B'He'elom Echod L'Rishus Echad (Shabbos 80a)

Rabbeinu Tam agrees with the Rach that the girsa is B'He'elom Echod Mai'Rishus Echad Chayov. What is the reason according to Rabbeinu Tam that Mai'Rishus Echad is required but not L'Rishus Echad? Rav Yaakov Kaminetsky z"l explains that if the two Chatzi Grogeres started in two different places and ended up in the same place, at the beginning they were nothing since they were not combined, only at the end did they become something Choshuv - he was Molid a new thing when he brought them together. But, if they began together, and he moved them to two different places, he moved something choshuv. He moved half of it to one place and half to another place, so he is Chayov.

Then, he asks, what is the reason of Rashi who learns that L'Rishus Echad Chayov? He explains that according to Rashi, the shiur of K'Grogeres is different from other shiurim of M'lacha on Shabbos. Other shiurim are about what is considerd Choshuv, less than the shiur is not Choshuv. However, the shiur of K'Grogeres is Halacha L'Moshe M'Sinai, so even though a smaller amount could be Choshuv, still the HL"M tells us that this is the shiur. Therefore, even though the two Chatzi Grogeres started in two different places, still they were Choshuv at the beginning. So, moving them to one place is not considered making a new thing (like Rabbeinu Tam), it is just considered to be moving two half shiurim and is mits'taref. He says that Rashi in other places can also be explained using this idea.