Tosafos writes that we need a pasuk to teach that eidim zom'min pay "idyis" (from their best) even when they tried to make someone pay only ziburis. But, in a case where they try to make someone pay idyis, it is obvious that they must also pay idyis and not ziburis, because that is implicit in the requirement to pay "ka'asher za'mam" - like they intended to do. My question on Tosafos is that we should really need a pasuk requiring eidim zom'min to pay idyis, even when they were trying to be mechayev someone to pay idyis. We pasken that the chiyuv of "mei'tav" is to pay the idyis of the mazik - meaning, that if the damager has property that exceeds the standard "best" of the world, the mazik must pay from his idyis. In a case where the eidim zom'min have a quality that exceeds the standard "best", even when they try to make someone pay a standard "best" (which is their highest quality field), the din of ka'asher za'mam requires them to pay only the standard best, but the din of "meitav" should require them to pay their best. Why does Tosafos say that we don't need the pasuk of "meitav" for a case where the eidim zom'min are trying to be mechayev idyis, we need a pasuk even for that case when they have a quality that exceeds idyis?
Perhaps Tosafos holds that even the din of ka'asher za'mam is mechayev to pay from their own best. Meaning, the eidim zom'min must pay like they tried to do. Did they try to make someone pay quality A (which was his best) or did they make someone try to pay "their best" so the ka'asher zam'am would require the eidim zom'min to pay their best which would be quality A+. Perhaps the din ka'asher za'mam is like the latter, in that they tried to make someone else loose his best (quality A) and therefore would have to pay their best (quality A+), even without a special pasuk of mei'tav. That is why Tosafos says we only need a pasuk of mei'tav for a case where they try to make someone pay ziburis, but not for a case where they try to make someone pay idyis.