Demai 5:5-6

Demai 5:5

If a person purchased food from a needy person, or if a needy person was given pieces of bread or of fig cakes, each piece must be tithed individually because they didn’t necessarily all come from the same source. When it comes to dates and dried figs, one may mix them together and tithe the mixture. Rabbi Yehuda says that one may only do this when the needy person was given a large gift but if he was given many small gifts, each must be tithed individually.

Demai 5:6

If one bought food from a wholesaler (as opposed to the farmer), and he then went back and purchased more, he may not take tithes from one purchase for the other because they may have come from different sources. This is true even if they came from the same bin and are of the same type. However, the wholesaler is believed if he says that both purchases came from the same source.