Demai 3:4-5

Demai 3:4

If a person takes his already-tithed wheat to a miller who is a Samaritan (quasi-Jewish and not reliable when it comes to tithes and Sabbatical produce) or to a miller who is unlearned, the status of the grain is unchanged. (We may not be able to trust them about tithes but we don’t suspect that they might switch tithed grain for untithed grain.) If he took it to a non-Jewish miller, what he gets back is demai and must be re-tithed out of doubt. If one gives his produce to a Samaritan or an unlearned person to watch, its status is unchanged when he gets it back. If he gives it to a non-Jew to watch, what he receives back must be treated the same as the non-Jew’s own produce and it must be tithed; Rabbi Shimon says it is demai and must be tithed out of doubt. (The practical difference is that, in the case of demai, he need not give it to a kohein because of the doubt. Rather, the owner may sell it to a kohein.)

Demai 3:5

If one gives an innkeeper ingredients to prepare for him, he must tithe both what he gives her and what he receives back. This is because we suspect that she may exchange his ingredients for other produce. Rabbi Yosi says that if an innkeeper does make such an exchange, it’s in order to cheat the customer. Since we don’t have to protect crooks from sin, the customer need only tithe what he gets back.