Rashi says that until the loan took place, the land was in the Reshus of the Loveh (borrower). Now that the loan has taken place, and the land has become mortgaged to the loan, it is no longer considered in the Reshus of the borrower. This is despite the fact that the borrower is still the owner of the land. (The reason for this is that, once the land is mortgaged the owner no longer has full control over his property, since he may be forced into relinquishing the property in order to service his debt.) This is what Rashi is telling us when he says that the sale or the Hekdesh of the borrower does not take effect because the land is "me'Mushkan" to the lender. Rashi explains that this is due to the principle that someone can only transfer ownership of a property if it is considered to be in his Reshus. Ownership alone is not sufficient. We learn this from the Pasuk of "Ish Ki Yakdish Es Beiso". When Rashi says "v'Af Al Gav she'Hein Shelo, Einan b'Reshuso", he means that even though they belong to the Loveh, they are not in his Reshus, as we explained above.

Dov Freedman