Baba Basra 94b - Passul Witnesses

In the sugya discussing imposing penalties to prevent people from doing issurim, the gemara discusses collecting the principal in a shtar that has ribbis whether we impose a penalty to prevent the lender from collecting even the principal. Tosafos asks, how can the Rabbonon hold that we use the contract to even collect the principal from property on which he has a lean (where the shtar itself is being used to collect), since the witnesses who signed in the contract are passul eidim for doing an aveira of signing a contract that would enable the collection of interest? Tosafos answers that witnesses tend to assume the issur of ribbis is only on the borrower and lender, but they don't realize that their is an issur on the eidim as well, so they don't become passul. A second answer is that we are speaking about a case where their lives were being threatened so that they don't become passul for signing. Tosafos in Baba Metzia offers another answer, that although by an issur d'oraysa one who does an issur not intending to profit would be passul, on an issur d'rabonon he would only be passul if he is doing it for monetary gain. Therefore, we are speaking about a contract where the ribbis is d'rabbonon, so that the witnesses who have nothing to gain would remain kasher witnesses.

This answer of Tosafos is paskened in the Rama (C.M. 34:3), that one who violates an issur d'rabonon is passul l'eidus on a rabbinic level, but only if it is a type of aveirah where there is a monetary gain. Meaning, the violation of an issur d'oraysa for any reason labels the person as a rasha (according to what we pasken), but the violation of an issur d'rabbonon doesn't label him as a rasha, just that we are concerned that he will lie in court for monetary gain, as he has done in the past (sm"a s.k. 5).

But in truth, the shulchan aruch (4) paskens that those who bury the dead on yom tov, violating yom tov, aren't passul since in their own minds they think that they are doing a mitzvah. The main point is not so much that they think they are doing a mitzvah, but rather that since in general their is a mitzvah involved, they don't consider their actions to be an aveirah. This is really the answer of our tosafos, that the witnesses don't consider the signing of a contract that has ribbis to be an aveirah. Being that the Shulchan Aruch accepts this approach, he would be willing to accept the answer of tosafos by us and therefore has no reason to be mechadesh as Tosafos does in baba metziah that an aveira d'rabonon only passuls when there is monetary gain. Furthermore, the gr"a points out that the opinion of the Rambam is that a witness will only be passul if he violates a la'av that has a ma'aseh (as shulchan aruch writes in si'if 2), therefore a ribbis vioation which is nituk l'aseh of וחי אחיך עמך, has no malkus, therefore the shulchan aruch will be able to answer tosafos question very simply by saying that signing in a contract of ribbis will not passul the eidim, even for ribbis d'oraysa. So, there is no reason to be forced to answer like tosafos in baba metziah and say that an aveira d'rabonon requires monetary gain to be passul.