i learned that if one makes a condition against the tora it is null and void.in the mishna in bechoros, how can someone make "kablanus" nosen bi-kablanus and be exempt from bechor? isn't that a t'nai against the tora?
looking forward to your reply
When the Gemara mentions, in a number of places (see Bava Metzia 94a, Kidushin 19b), that when one makes a stipulation against what is written in the Torah his stipulation is null and void, this means that it is not possible to make the stipulation that the sale (or Get, or Kidushin, etc.) take effect only on condition that a Mitzvah of the Torah be transgressed.
In the Gemara in Bechoros, however, the situation is different. There is no prohibition involved with the sale itself (nor is there any Tenai). Rather, because the Nochri has a portion in the Bechor, then automatically there is an exemption from the Mitzvah of Bechor, and thus no transgression is being committed with the sale. (This is similar to a person who did not want to eat bread in a Sukah during Sukos after the first night of Sukos; such a person is "Patur" from sitting in the Sukah, and he has not transgressed any Isur.)