The bible teaches us that we pay a tenth on any INCREASE that we receive. There is a free book which speaks on money and god as well as on tithing. You can download a free copy of the book at www.financialwisdomkeys.com
We pay tithe on any increase of income received. One question is if we pay on the Net or Gross. If we pay on the gross income that covers all of the tithe. If we pay on the net then receive a refund we must pay a tithe on that (increase).
If you go the website Biblegateway.com and type in Malachi 3:10 and you can see what God says about tithing. When we tithe it has to be the very first 10 percent off of your income or money. You will see Gods blessings like I have. Creditors stopped calling me. I had extra money in the bank and didn't know where it came from. All of my bills were paid.It was awesome. People gave all kinds of money and favors, clothes, toys, school books, knapsacks, food, etc...You'll see. God says.." I will pour you out a blessing, you shall not have room enough to receive". It is real and it works.