Brachos 47a: If a Levi received Ma'aser Rishon from the ears of corn (before the winnowing has taken place) 1 , it is exempt from Terumah Gedolah. If he received Ma'aser Rishon from the Kri (the pile of grain), which is already obligated in Terumah Gedolah, he must give also it.
R. Shimshon inferred that if a Levi received Ma'aser Rishon from sheaves, it is proper Ma'aser, since we need "Ma'aser Min ha'Ma'aser" to exempt it from Terumah Gedolah (Brachos 47a). Perhaps it is exempt, and the verse exempts one who received Ma'aser Rishon from a Kri!
Da'as Zekenim (Vayikra 27:30): If Ma'aser Rishon from the ears of corn was invalid, and the verse exempts one who received from a Kri, we could not fulfill (verse 28) "mi'Kol Matnoseichem Tarimu"!
If one separated Ma'aser from sheaves, before it is called Degen, it is Ma'aser (Brachos 47a). If one separated Chalah before kneading, it is not Chalah!