מתנה על מה שכתוב בתורה

The Gemara made an initial assumption when it saw Shmuel's ruling regarding the Shemitta year; in the case, a lender made a תנאי with the borrower - to the effect that the Shemitta would not cancel a loan.  Shmuel rules that this תנאי  is בטל - or invalid. The Gemara assumes that this is called מתנה על מה שכתוב בתורה and that when one is מתנה, the תנאי is בטל or invalid.

The Gemara raises a parallel case in אונאה - which is cheating another person in a business deal. There, Rav rules that a תנאי that makes it not subject to אונאה doesn't work, while Shmuel rules that is does work.

This seems to be an internal contradiction in Shmuel; here - he seems to hold מתנה ע"מ שכתוב בתורה תנאו קיים !

Rav Anan reports that Shmuel once made the following distinction for him:
If the language of the request is to have a particular monetary right waived, then the תנאי works; if the language of the deal tries to make a תנאי that either Shemitta does not cancel loans or אונאה does not exist in a case of overcharging - then such a clause is not a halachic תנאי - and is invalid.

Coming out of this: Although Shmuel rules that מתנה works, the earlier case is NOT ONE OF מתנה - and that's why it doesn't work!!!