Bechorot 27 - Kohen Helping on the Threshing Floor
If a Kohen says to a Jew who is caring for a firstborn , "Give it to me before the prescribed time; you will have it off your hands, and I will now care for it" - the Jew is not allowed to do it. Why not? Because that kohen will appear as working for a reward. The firstborn animal is a gift, and it must be given freely and in a dignified manner. This is similar to a Kohen who would help on the threshing floor to later get his portion (terumah), or a Levite in a like situation, to get his tithe, and the Torah said, " Do not profane the holies of the Children of Israel, so that you will not die ."
So that the laws of terumah, or the Kohen's portion, were not forgotten, the Sages instituted it to be given in lands close to Israel even nowadays, but they made its laws more lenient. For example, it is nullified in a simple majority of regular produce, not in 1 in 100. There are similar leniencies regarding challah, the kohen's portion from dough, which the Sages required to give in all lands.
Art: Albert Dubois-Pillet - Lady Carrying Bread