When it comes to tithing it is not our decision on whether it's on the gross income or net - it's God's. God has said many times throughout the Bible that we must honour him with the first 10 percent of our profits. He doesn't say to pay everyone that we owe money to and then take the 10 percent off of that - no the Bible clearly states that it is to be the FIRST TEN PERCENT. In biblical times it was not necessarily money that people tithed, it was often the first 10 percent of their harvest. An example of this is found in Deuteronomy 14:22 it says "Be sure to set aside a tenth of all that your fields produce each year." This verse makes it clear when it says ALL that your fields produce. If tithing was based on your net income this verse would not have the word all in it but it would say 'set aside a tenth of what your fields produce after thrashing and feeding your family and trading for taxes'.
When we tithe with a heavy heart, meaning we really don't want to give but we do because we know we should or when we give 10 percent of our net income we are not giving with a joyful heart - which is just as bad as not giving at all. When we give God what is his he will bless us greatly. Often it's hard to give away our money knowing that we have so many bills to pay and items needed for our house or children or parents but that is where our faith must come into play. We must have faith that God will provide us with everything we need as long as we honour him and the way to do that is through tithing. In the Old Testament tithing was done to help support the priests, now we give to help support the churches needs but the real reason behind tithing is to show God that without him we would have nothing. God does not need the money and we know that but can we still give with a joyful heart and give our best (the first 10 percent) and do God's will?
So next time you drop your money in the collection plate remember to do so with a happy heart, give your first 10 percent and know that God will take care of the rest!