Demai 5:7-8

Demai 5:7

If a person buys produce from an unlearned landowner, and he then buys more produce from him, the purchaser may take tithes from one batch for the other. This is true even if the landowner sold to him from different bins – or even from fields that he owns in different cities! If the landowner was selling vegetables in the marketplace and they all come from his gardens, the purchaser may tithe from one batch on another. If the vegetables come from other gardens, then the customer must tithe each purchase separately.

Demai 5:8

If a person buys tevel (definitely untithed produce) from two sellers, he may take tithes from one for the other. This is so even though the Sages said that tevel may only be sold in extenuating circumstances.