It is THE eternal debate in the church:
"Should we pay net or gross tithing?"
I can understand why people are so concerned about this very pressing issue. For most people, money is tight. The difference between gross and net tithing can mean the inclusion or exclusion of a variety of many things to an individual or family.
Let's turn to the scriptures, and see what they say, shall we?
On July 8, 1838, Joseph Smith requested of the Lord, "O Lord! Show unto thy servants how much thou requirest of the properties of thy people for a tithing." The result was D&C Section 119. In it, the Lord answered two primary questions: How much are Saints expected to pay in tithing, and what is the money to be used for?
The second answer was straightforward: Tithing money is to be used for (1) the building of a temple, (2) laying of the foundation of Zion and for the priesthood, and (3) the debts of the Church's Presidency.
Nowhere in D&C 119 is the church allowed to invest its funds. This would be in contradiction to the words of God, where we're instructed to "lay not up for yourselves treasures upon earth, where moth and rust doth corrupt." 
That's not all. D&C 119 makes no stipulations for the funding of:
- Meetinghouses 
- Humanitarian work 
- Missionary efforts 
- Broadcast facilities
- Agricultural reserves
- Real estate
Neither does it make provision for funding full-time salaries. It's perfectly natural that these are not funded, because Jesus' original apostles worked without purse nor scrip, yet still had living expenses and even taxes to pay (to Rome). Why are apostles today any different? Here is what the Lord said:
"Therefore, ye shepherds, hear the word of the LORD;
As I live, saith the Lord GOD, surely because my flock became a prey, and my flock became meat to every beast of the field, because there was no shepherd, neither did my shepherds search for my flock, but the shepherds fed themselves, and fed not my flock;
Therefore, O ye shepherds, hear the word of the LORD;
Thus saith the Lord GOD; Behold, I am against the shepherds; and I will require my flock at their hand, and cause them to cease from feeding the flock; neither shall the shepherds feed themselves any more; for I will deliver my flock from their mouth, that they may not be meat for them." 
But I digress. Perhaps a more thorough examination of these outlays may be appropriate in a subsequent post.
"GROSS OR NET?" THROUGHOUT HISTORY
The first response the Lord provided to Joseph addressed how much He expected the Saints to offer. The answer was also straightforward: (1) offer all their surplus property into the hands of the bishop of my church in Zion, and (2) "one-tenth of all their interest annually" thereafter. To ensure it stayed this way, He suffixed his command by saying, "and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord." 
Elsewhere, the Lord defines "surplus" as the part of one's income that is "more than is necessary for their support." 
OK, fine. But what is one's "increase"?
The Lord's insistence that tithing be based on interest is consistent with what He told Abraham to do:
"Abram paid unto him [Melchizedek] tithes of all that he had, of all the riches which he possessed, which God had given him more than that which he had need." 
So, did Abraham pay tithes based on his gross income? No. Did Abraham pay tithes based on his net income? No. He paid tithing on "more than that which he had need," which sounds a lot like "more than is necessary for their support." In other words, his surplus.
At the time D&C 119 was revealed, John Corrill was serving alongside Elias Higbee as Church Historian. This is how he described the implementation of tithing:
"If a man gives for the benefit of the Church, it is considered a voluntary offering. Yet the law requires or enjoins a consecration of the overplus, after reserving for himself and family to carry on his business." 
"Overplus," which is defined as - you guessed it - "a surplus or excess."
In 1855, Brigham Young reminisced about how tithing was implemented "When the revelation which I have read was given in 1838, I was present, and recollect the feelings of the brethren. . . . The brethren wished me to go among the Churches, and find out what surplus property the people had, with which to forward the building of the Temple we were commencing at Far West. I accordingly went from place to place through the country. Before I started, I asked brother Joseph, 'Who shall be the judge of what is surplus property?' Said he, 'Let them be the judges themselves. . . .'" 
There's that word "surplus" again!
A CHANGED "STANDING" LAW
The first liberalization of the 1838 tithing revelation was instituted by the Quorum of the Twelve Apostles in November, 1841. Then, the initial donation was reduced to only "one-tenth of all a man [possesses, and] 1/10 of increas[e] afterwards." 
(You'll note the usage of the word "increase" instead of "interest," which are considered synonyms).
I've read no objections or commentary by Joseph Smith regarding this modification, so I would assume that it still didn't violate the Lord's standing law.
But two and a half years later, tragedy struck: Joseph Smith, Jr. was martyred on June 27, 1844.
And that's where the story gets interesting.
On August 15th, 1844, just six weeks of Joseph's martyrdom, the Quorum of the Twelve Apostles issued an epistle, (signed by Brigham Young, President of the Twelve) which entirely did away with surplus-based tithing and replaced it with income-based tithing:
"Therefore, as soon as the Twelve have proceeded to a full and complete organization of the branches abroad, let every member proceed immediately to tithe himself or herself, a tenth of all their property and money, and pay it into the hands of the Twelve; or into the hands of such bishops as have been, or shall be appointed by them to receive the same, for the building of the Temple for the support of the priesthood according to the scriptures, and the revelations of God; and then let them continue to pay in a tenth of their income from that time forth, for this is a law unto this church as much binding on their conscience as any other law or ordinance. And let this law or ordinance be henceforth taught to all who present themselves for admission into this church, that they may know the sacrifice and tithing which the Lord requires, and perform it; or else not curse the church with a mock membership as many have done heretofore. This will furnish a steady public fund for all sacred purposes, and save the leaders from constant debt and embarrassment, and the members can then employ the remainder of their capital in every branch of enterprize, industry, and charity, as seemeth them good; only holding themselves in readiness to be advised in such manner as shall be for the good of themselves and the whole society; and thus all things can move in harmony, and for the general benefit and satisfaction of all concerned." 
Five months later, in January 1845, another Quorum of Twelve's epistle highlighted "the duty of all saints to tithe themselves one-tenth of all they possess when they enter into the new and everlasting covenant: and then one-tenth of their interest, or income, yearly afterwards." 
Then, two weeks later, the Twelve voted to exempt themselves, the two general bishops (Newel K. Whitney and George Miller) and the Nauvoo Temple Committee from any obligation to pay tithing. 
We are told that "The Twelve are a Traveling Presiding High Council, to officiate in the name of the Lord, under the direction of the Presidency of the Church, agreeable to the institution of heaven; to build up the church, and regulate all the affairs of the same in all nations, first unto the Gentiles and secondly unto the Jews.".  It is written: President [Joseph] Smith gave the following decision. They are the Twelve Apostles, who are called to the office of traveling high council, who are to preside over all the churches of the Saints among the Gentiles, where there is no presidency established, and they are to travel and preach among the Gentiles, until the Lord shall command them to go to the Jews. They are to hold the keys of this ministry, to unlock the door of the kingdom of heaven unto all nations, and to preach the Gospel to every creature. This is the power, authority and virtue of their apostleship." 
To reinforce this fact, "The president then stated that the Twelve will have no right to go into Zion or any of its stakes and there undertake to regulate the affairs thereof where there is a standing High Council." 
Today, "In addition to their primary responsibility to be special witnesses of the name of Christ throughout the world, the apostles have heavy administrative responsibilities as they oversee the orderly progress and development of the global Church." 
Based on the aforementioned facts, I have the following questions:
- Joseph Smith told a gathering of the Twelve that they only had authority "where there is no presidency established,", and in fact were not even to go into any organized stakes where there is a standing High Council. But now they have administrative responsibilities over the entire global Church. Where is the revelation that supersedes Joseph Smith's instructions, and when was it presented for a vote to the church as a whole per the Law of Common Consent?
- Tithing was pretty clearly defined in the scriptures and statements by Joseph Smith that it's based on one's surplus, not income. Where is the revelation (the basis for the 8/15/1844 epistle) that supersedes those scriptures (including the Lord's standing law to the church) and Joseph Smith's instructions, and when was it presented for a vote to the church as a whole per the Law of Common Consent?
- What does "a standing law forever" mean to you?
- Where is the revelation that exempts the Twelve and others from having to pay tithing? When was it presented for a vote to the church as a whole per the Law of Common Consent?
- If we are to receive profound blessings for paying tithing, then aren't those blessings naturally withheld to the Twelve, who are not obligated to pay tithing?
- If Joseph were asked if tithing should be based on gross or net income, what do you think he'd say (and what would be the factual basis for your answer)?
CHECK THE COMMENTS BY LATTER-DAY
TRUTHS BELOW FOR UPDATES TO THIS POST
1. 3 Nephi 13:19-20; Helaman 8:2
2. "The vast majority of these funds are used immediately to meet the needs of the growing Church including more meetinghouses, temples, education, humanitarian work and missionary efforts throughout the world. All Church funds exist for no other reason than to support the Church's divinely appointed mission." (First Presidency statement, Dec. 17, 2019)
3. Ezekiel 34:7-10
4. D&C 119:4
5. D&C 42:33
6. JST Genesis 14:39
7. Scott G. Kenney. "John Corrill (1794-1840)". Saints Without Halos. Retrieved 2010-08-12; Susan Easton Black (1997). "Corrill, John". Who's Who in the Doctrine and Covenants. Salt Lake City: Bookcraft.
8. John Corrill, "A Brief History of the Church of Christ of Latter Day Saints", published in 1839. Reprinted in its entirety in The Joseph Smith Papers, Volume 2-The Histories
9. Journal of Discourses 2:306; http://jod.mrm.org/2/298
10. Meeting of seven members of the Quorum of the Twelve Apostles with English immigrant Joseph Fielding at Nauvoo, Illinois, 31 Nov. 1841, minutes, archives, Historical Department, The Church of Jesus Christ of Latter-day Saints. Salt Lake City, Utah; Arrington, Great Basin Kingdom, 18, and Swainston, "Tithing" in Encyclopedia of Mormonism 4:1482 overlook this as a liberalization of the requirement in the 1838 revelation.
11. Joseph Smith et al., History of the Church of Jesus Christ of Latter-day Saints. Period I: History of Joseph Smith, the Prophet, and. . . Period II: From the Manuscript History of Brigham Young and Other Original Documents, ed. B. H. Roberts, 7 vols. (Salt Lake City: The Church of Jesus Christ of Latter-day Saints. 1902-32; 2d ed. rev. [Salt Lake City: Deseret Book Company, 1978]), 7:251; Arrington, Great Basin Kingdom, 18; http://www.boap.org/LDS/History/History_of_the_Church/Vol_VII; https://archive.org/stream/historyofchurcho03smitrich/historyofchurcho03smitrich_djvu.txt
12. History of the Church, 7:358
13. Heber C. Kimball diary, 29 Jan. 1845, in Stanley B. Kimball, ed., On the Potter's Wheel: The Diaries of Heber C. Kimball (Salt Lake City: Signature Books/Smith Research Associates, 1987), 94; Nauvoo Trustee-in-Trust Tithing and Donation Record, 220-222 (29 Jan. 1845), LDS archives. For the term general bishop and its meaning in early LDS history, see Journal of Discourses, 26 vols. (Liverpool and London: Latter-day Saints' Book Depot, 1854-86), 22: 34 (O. Pratt/1880); D. Michael Quinn, 'Evolution of the Presiding Quorums of the LDS Church,' Journal of Mormon History 1 (1974): 34; Dale Beecher, 'The Office of Bishop,' Dialogue: A Journal of Mormon Thought 15 (Winter 1982): 103; Quinn, Mormon Hierarchy: Origins of Power, 69-71
14. D&C 107:33
15. "Minutes and Discourses, 27 February 1835, as Reported by Oliver Cowdery," in Minute Book 1, 88, https://www.josephsmithpapers.org/paper-summary/minute-book-1/92 ; as quoted in https://www.churchofjesuschrist.org/study/history/topics/quorum-of-the-twelve
16. Minute Book 1,p. 187, May 2, 1835; https://www.josephsmithpapers.org/paper-summary/minute-book-1/191#source-note
17. Church news release, "Russell M. Nelson: New President of the Quorum of the Twelve Apostles", July 15, 2015, https://newsroom.churchofjesuschrist.org/article/russell-m-nelson-new-president-of-the-quorum-of-the-twelve-apostles