The bible mentions giving a tenth throughout the old testiment and in the new testiment also, just not as often.
And no, God does not keep a balance book but he knows what is in our hearts:
2 Cor 9:7 (Jesus said
Each man should give what he has decided in his heart to give, not reluctantly or under compulsion, for God loves a cheerful giver.
You can twist it around anyway you want but the whole point is that man should care about his fellow man and take care of one another. Sharing wealth with those less fortunate is one example. I can imagine that some folks think that is a sucker's belief, but we are judged by how we care.