The current version of the Against the Bot rules do not allow salvage rights to be renegotiated. Earlier versions did, and I don't remember when it changed. The original AtB code in MekHQ was based on 2.25, but was updated to 2.29, and it's possible that salvage renegotiation was there in a much earlier version, but if so it was years ago. The FMM(R) setting is for determining unit rating and not the entire system, with is a conglomeration of FMM, an early Campaign Operations pre-release version, and a whole lot of custom rules.