משנה אויף שמות 22:28

If a man bought [grain from an am haaretz] to be used for seed or for animal [feed], flour for hides, oil for a lamp, or oil for greasing utensils, it is exempt from [the rules of] demai. [Produce grown] beyond Cheziv and north is exempt from [the rules of] demai. The hallah of an am haaretz, produce mixed with terumah, produce bought with second tithe money, and the leftovers of minhah offerings are exempt from [the rules of] demai. Oil spiced [with spices from an am haaretz]: Bet Shammai makes it liable [to the rules of demai]. But Bet Hillel exempts it.

He who gives terumah before first-fruits, or first tithe before terumah, or second tithe before first tithe, although he transgresses a negative commandment, that which he has done is done, for it is said: “You shall not delaying the skimming of the first yield of your vats” (Exodus 22:28).

If orphans relied on a householder or if their father appointed a guardian for them, he must tithe their produce. A guardian who was appointed by the father of the orphans is required to take an oath. [A guardian who was] appointed by the court does not need to take an oath. Abba Shaul says that the rule is the reverse. One who renders impure [someone else’s pure food] or mixes terumah [with someone else’s non-terumah produce] or makes a libation [with someone else’s wine], if he does so inadvertently, he is exempt, but if intentionally he is liable. Priests who intentionally made someone else’s sacrifice piggul in the Temple are liable.