http://www.wheatandtares.org/19945/tithing-have-you-considered-paying-on-surplus/
Wheat & Tares welcomes guest poster Bill Reel: Host of“Mormon Discussion Podcast”. His Podcast serves to help those reconstructing their faith within Mormonism to do so “Leading with Faith” in the gospel rather than leaving the Church.
D&C 119 renews tithing in this dispensation as a commandment and hence every week Latter-day Saints grab envelopes next to their Bishop’s office and/or sign into LDS.org to pay their tithing on 10% of their income. Yet have you ever stopped to ask what is income? Some people pay on gross, others on net, and a few pay on surplus. Wait, you didn’t know surplus was an option? Well, it is.