A seventy-year-old man became a baal teshuvah and had never given maaser. He calculated that he owed approximately $200,000 and he only had $30,000 in his bank account. If the halacha calls for him to pay the maaser, he was willing to use all of his savings! Rav Moshe Shternbuch (Shut Teshuvos Vehanhagos 2:483) paskened that he need not pay past maaser and that he should only give 10% of his $30,000 to tzedaka for the following reasons.
1. Maaser on monetary earnings is only a minhag. Even according to those that it is Min HaTorah, it is only when the earnings are before you and not when it has already been spent.
2. The mitzvah of maaser is a wise investment. It is a way of bringing Hashem into your business as a Partner. With a Partner like Hashem, the investment is bound to bare fruits. Once the money is already spent, this is not relevant.
3. The halacha is that those who charge ribbis (interest), when they repent and want to return the money, the borrower should not accept the money because this will make it easier for sinners to do teshuva. In this case, there isn’t even any specific person to return it to.
Rav Shternbuch praises the questioner and says fortunate is the nation that serves their Master and fortunate is the King who has children like this.