The IRS uses your gross as a basis. How much more the God that gave you the health and ability to work in the first place? :)
"Offerings" are over and above the tithe. You can never out-give the Lord :)
Bottom line. God doesn't need your money. But, he wants us to rend our hearts. If your heart is right with God, this is just one more of His commands that you will be obedient to observe :)