3,437. Taking Tithes From One Basket for Another

Hilchos Maaser 7:11

Let’s say that there are two baskets of untithed produce in front of a person and terumah has already been taken from them. If the person says, "Let the tithes for basket A be in basket B," his tithing is effective. If he says, "Let the tithes for basket A be in basket B and the tithes for basket B be in basket A," only the tithing of basket A is effective because he established that its tithes are in the basket B. The tithing of the basket B fails because tithes can’t be taken from basket A. This is because one can't take tithes from produce whose obligation has already been fulfilled.

Hilchos Maaser 7:12

If the person says, "As far as the tithes for these two baskets of produce, let the tithes for each be in the other," he has effectively designated the tithes. He must therefore take the tithes from these baskets and not from elsewhere.