Terumos 3:6-7

Terumos 3:6

If a person takes terumah before first fruits, or first tithe before terumah, or second tithe before first tithe, even though he violated a prohibition by going out of order, what he did is valid. We see that going out of order is a prohibition from Exodus 22:28, “From your abundance and your liquor you shall not delay.” (It is inferred that abundance refers to first fruits, which only come from produce, while liquor refers to terumah, which also comes from liquids.)

Terumos 3:7

How do we know that first fruits must be taken before terumah, since the Torah calls each of them terumah (a gift) and reishis (the start)? First fruits come first because they become obligated first, while the produce is still attached to the ground. Terumah comes before first tithe because the Torah calls it “reishis” (the start). First tithe comes before second tithe because it contains the terumas maaser, which is terumah and therefore reishis.