Terumot, Chapter Four, Mishnah Four
One can appoint a messenger (or agent) to separate terumah on his behalf. The question that our mishnah asks is, how much terumah should the messenger take out?
1) One who says to his messenger: Go and give terumah [for me], he should give terumah in accordance with the mind of the owner.
2) If he does not know the mind of the owner, he gives according to the average amount one fiftieth.
3) If he gave ten parts less or more, the terumah is terumah.
4) If, however, his intention was to add even one part more, his terumah is not terumah.
Section one: If the messenger knows how much terumah the owner of the produce usually gives, then he should give that amount.
Section two: If he doesnt know, then he should give an average amount.
Section three: If he gave ten parts more or ten parts less, meaning he gave one fortieth or one sixtieth, it is still valid because these are all valid amounts of terumah.
Section four: However, if he knew how much the owner usually gives, and he intentionally added even one part or gave even one part less, then his actions are competely invalid. For instance, if the owner usually gave one fiftieth, and the messenger intentionally gave 1/51 or 1/49 his terumah is not terumah at all.