Terumot, Chapter Four, Mishnah Two
If one had his produce in a storehouse, and he gave a seah to a Levite and a seah to a poor person, he may set aside another eight seahs and eat them, the words of Rabbi Meir.
But the sages say: he may only set aside according to proportion.
A person has a storehouse of produce from which tithes have not been taken out. He sees a Levite and he gives him one seah of his produce and then he sees a poor person and gives him one seah of his produce. According to Rabbi Meir, he may then go to his produce and eat another 8 seahs, with the understanding that the seah that he gave to the Levite was first tithe and the seah that he gave to the poor person was poor persons tithe (given in the third and sixth year of the sabbatical cycle). The remainder of his produce remains totally untithed.
The sages do not allow this because they say that the two seahs he gave count towards the entire storehouse, which contains far more than eight other seahs of produce. Any seah that he removes from the storehouse is only partially tithed for, according to the percentage of produce that the two seahs that he did give would account for. For instance, if he had one hundred seahs in the storehouse, each seah that he takes out is only ten percent tithed (he removed two seahs and he should have taken twenty). Before he eats any of the remaining produce he must complete the tithing process.