I did not get a chance last week to finish the topic of safeik shtar mukdam. Tosfos proved from Sanhedrin 32 that if eidim are challenged that they were not around on the date in a shtar we assume the shtar was post-dated rather than assume they are eidim zomimin. Based on this premis, Tosfos asks why we need a takanah to establish a rosh hashana to count years of a king’s reign. The worst case scenario (according to Rashi) is we won’t know whether the shtar is pre-dated and pasul or post-dated and kosher - just like we don’t assume eidim are zomimim if the shtar might have been post-dated, so too, in Rashi’s case, why assume the shtar is pasul if it might have been post-dated?
The simple answer to Tosfos question is that while it is true that we can validate the shtar even in a case of doubt, it is far better to avoid such a situation. Therefore , Chazal made the takanah of a set date to count years by.
The Brisker Rav offers a deeper explanation. Recall that according to Rashi, a shtar mukdam is pasul only viz. collecting from lekuchos; the creditor is still able to collect from other assets of the debtor. Tosfos disagrees and says the testimony of the shtar is completely invalid. Perhaps Rashi can be explained l’shitaso. In the sugya in Sanhedrin, the issue is whether to reject eidim completely and assume they were zomimim or to assume a shtar was post-dated – given the choice of completely invalidating testimony or not, we grant the shtar a presumptive chezkas kashrus. However, in the case in Rosh haShana, the eidus of the shtar that a loan took place is valid whether we assume the shtar is mukdam or not. The only issue is whether the creditor has an additional right to collect from lekuchos. Since this added right is not part of the crux of the testimony inherent in the shtar which simply established that a loan took place, the question of whether the shtar is mukdam or me’uchar should be treated as a safeik mamom and the creditor would lose, if not for the takanah.