Commentary for Kiddushin 123:11

The Talmud discusses a mishnah from elsewhere because our mishnah will be brought up as part of the discussion. Terumah cannot be separated from detached produce in order to exempt produce still attached to the ground. R. Assi then asks what if someone takes some detached produce and declares that the terumah he is separating will count as terumah for the attached produce when the attached produce becomes detached from the ground. R. Yohanan says that this works. Since he can detach the produce from the ground, we treat it as if it was already detached.