Kesubos 18a - Ein Adam Mei'iz

Tosafos explains that when Reuven claims $100, if Shimon denies it all he is believed simply because Reuven who is coming to take from Shimon bears the burden of proof. Rabba is bothered why does the torah say that if shimon agrees to $60, he must swear that he does not owe the $40, shimon should not have to swear since he has a migu that he could of denied the entire $100. To that the gemara explains that since denying everything is a tremendous chutzpah, there is no migu that he could have denied everything because even though he could have, he wouldn't have (The gemara then continues by saying we should still have a migu since it is also a chutzpah to deny partially so he would have just as easily denied the whole thing, and to that responds that to deny partially is not a chutzpah since he is not intending to deny, rather to delay and pay later).

Tosafos quotes Rashi from Baba Kamma but seems to quote it somewhat differently than rashi actually says. Rashi in B"k 107a says that the rationale for one not to deny the entire loan is that since the lender did him a favor, he would not be kofer tov and deny the loan. But, by a pikadon where the shomer (watchman) is actually doing a favor for the mafkid it is not a chutzpah to deny and therefore the shomer mut swear even if he denies the entire thing. Rashi is clearly understanding that the believability of kofer hakol is not simply based on hamotzi m'chaveiro.... but rather requires the chazaka of "ein adam mei'iz", since by pikadon that chazaka doesn't apply, the watchman is not believed. Based on this, it is very difficult to understand what Tosafos is asking on Rashi, that a modeh b'miktzas by pikadon should not be believed since he has a migu to be kofer hakol, since rashi himself says that even a kofer hakol is not believed. But, the difficulty with rashi is that if ein adam mei'iz is needed for kofer hakol to be believed, why when the son claims that he owed the father money do the rabbonon say that by the son where we don't have such a chazaka, you are believed even to be modeh b'mitkzas migu that you could deny everything; the opposite should be true, since there is no chazaka you should not even be believed to deny everything?