Dear Yair,

In order not to get mixed up, let's call the father (Hu Lifdos) B, his son (Beno Lifdos) C, and the father's father A.

Rashi, when explaining Rebbi Yehudah, says that to take care of C is B's responsibility, and B's Pidyon is A's responsibility, so B can redeem his son C first and leave the Shibud for B's own Pidyon. (Notice Rebbi Yehudah's words in the Beraisa.) The Maharsha explains: Even though Rebbi Yehudah maintains that Mitzvas ha'Guf comes first, that is only when one sum of money exists, but here there are two sums, so B should take care of C and the Shibud is left for B's own Pidyon. The Tana Kama argues that B's own Pidyon must be first, and there is nothing left for C.

Tosfos argues on Rashi and says that Rebbi Yehudah cannot say this against the Lekuchos, the new owners of the Shibud; its not fair to take from them.

Rebbi Yehudah is talking about when the land was owned by A and therefore it is already set aside for B's Pidyon and he should use the money for the Pidyon of C.

All the best,

Reuven Weiner