I think the scriptures show that the Jews under the Mosaic Law were to tithe but there was no penalty for it if they could not afford it. I also believe that at Jesus death, Jews and Proselytes that became Christians were released from the law of decrees and non-Jewish Christians were never under the law of Moses.
(Mark 7:10-13) For example, Moses said, ‘Honor your father and your mother,’ and, ‘Let the one who speaks abusively of his father or mother be put to death.’ 11 But you say, ‘If a man says to his father or his mother: “Whatever I have that could benefit you is corban (that is, a gift dedicated to God),”’ 12 you no longer let him do a single thing for his father or his mother. 13 Thus you make the word of God invalid by your tradition that you have handed down. And you do many things like this.”
(Matthew 15:4-6) For example, God said, ‘Honor your father and your mother,’ and, ‘Let the one who speaks abusively of his father or mother be put to death.’ 5 But you say, ‘Whoever says to his father or mother: “Whatever I have that could benefit you is a gift dedicated to God,” 6 he need not honor his father at all.’ So you have made the word of God invalid because of your tradition.
(Matthew 23:23) “Woe to you, scribes and Pharisees, hypocrites! because you give the tenth of the mint and the dill and the cumin, but you have disregarded the weightier matters of the Law, namely, justice and mercy and faithfulness. These things it was necessary to do, yet not to disregard the other things.
(Luke 11:42) But woe to you Pharisees, because you give the tenth of the mint and of the rue and of every other garden herb, but you disregard the justice and the love of God! These things you were under obligation to do, but not to disregard those other things.
(Luke 18:12) I fast twice a week; I give the tenth of all things I acquire.’
(1 Timothy 5:8) Certainly if anyone does not provide for those who are his own, and especially for those who are members of his household, he has disowned the faith and is worse than a person without faith.
(2 Corinthians 8:12) For if the readiness is there first, it is especially acceptable according to what a person has, not according to what a person does not have.
(2 Corinthians 9:6, 7) But as to this, whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. 7 Let each one do just as he has resolved in his heart, not grudgingly or under compulsion, for God loves a cheerful giver.
http://wol.jw.org/en/wol/d/r1/lp-e/2002881?q=%22Tithing+and+the+Law%22&p=par
