There is something in laws of Truma of the Rambam [chapter  1] that I have a hard time with. It comes from a well known statement of Reish Lakish that if one takes the first tithe before he takes truma from sheaves of wheat that have not yet been separated and grounded then the Levi does not have to take truma but only maasar. In that the Rambam is going like the Babylonian Talmud [Abyee], not like the Jerusalem Talmud. The Jerusalem Talmud holds that the Levi gives truma only if the tithe was taken after the crops became obligated. The thing that I find confusing is that in the first case the crops are tevel [obligated after the grinding] and the maasar is not even maasar. So why would the maasar be anything but a present/gift? The tithe was given before the crops even became obligated, so the maasar is not even maasar. In the case of the Jerusalem Talmud, the crops are actual tevel and still there is no obligation to give truma.

These are not really hard questions once you have verses which state that that is just the way it is. But what is odd is the Rambam says the reason in the first case is that the crops have not yet been obligated in truma. How is that a reason? In both cases you have real tevel that is not going to have truma taken from it. So on either Gemara I really have no question. It is rather the reason the Rambam gives that I find hard to understand.