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The Gemora has a principal that one does not receive lashes for a transgression that can be fixed up through the performance of a positive commandment.
Tosfos understands the Mishna to be saying that one would receive lashes for taking a millstone as a security, since the millstone contains two components, which are considered separate and independent parts regarding the violation.
Tosfos asks: Why should one receive lashes at all since the rationale behind the prohibition is that these are ochel nefesh type items (meaning they are needed for his livelihood), so it can potentially be fixed by returning them, so that there should not be lashes associated with this violation at all?
The proof that Tosfos cites for this being a “la’av hanitek l’aseh” (a transgression that can be fixed up through the performance of a positive commandment) and that one doesn’t receive lashes for such a transgression, is the story quoted on 116a where a person took a slaughtering knife as collateral and Abaye commanded him to return it. Tosfos at first understands that the only rationale for returning the item would be that it is a“la’av hanitek l’aseh”. Ultimately, Tosfos concludes that it is not a “la’av hanitek l’aseh,” and the only reason that Abaye demanded that it be returned is that the lender didn’t realize when he took it that it was forbidden, so that he never acquired it as a security, and therefore it had to be returned.
Regarding taking ochel nefesh type items as a security, there is an argument amongst the Rishonim. Tosfos 113a (d.h. v’es), holds that any item that is needed for livelihood may not be taken at all as collateral. However, the Maharsha quotes many Rishonim who disagree with Tosfos and hold that it may be taken as collateral, but must be returned when the borrower needs them to use for his livelihood. See also Hagahos HaGra on Tosfos who quotes that the Ramban and Rashba hold that it may be taken, but must be returned when needed, whereas the Rambam agrees with Tosfos that it may not be taken at all.
Now, the entire assumption of Tosfos that the reason Abaye must have insisted on returning the ochel nefesh collateral was because it is a “la’av hanitek l’aseh”, is following his own line of reasoning. Had Tosfos held like the Ramban and Rashba, there would be no proof at all from the story of Abaye because Abaye was merely telling the person that the standard rules of this type of security is that it must be returned when the borrower needs it.
Tosfos also assumes that if we would say “Whenever the Torah says not to do something, and one goes ahead and does it anyway, it is not effective,” then it would make sense that Abaye would demand returning the security, since he wasn’t allowed to take it, the taking was ineffective.
There is a big discussion in the Achronim (Chavos Da’as and R’ Akiva Eiger in Hilchos Shechita) whether “Whenever the Torah says not to do something, and one goes ahead and does it anyway, it is not effective,” applies when the prohibition will anyway not be fixed. Meaning that “if one goes ahead and does it anyway, it is not effective” may only apply when we say that by not taking effect, the prohibition will not have been violated. Based on that principal, it is a big novelty for Tosfos to assume that “if one goes ahead and does it anyway, it is not effective” would apply here. Even if the kinyan doesn’t take effect, there is certainly some transgression violated by taking an item of the borrower that should not be taken - if not for the prohibition of “do not take,” there would be a prohibition against stealing. Yet, Tosfos assumes that since “if one goes ahead and does it anyway, it is not effective” would help avoid “do not take as a security,” even though it will cause a prohibition of stealing, we can still apply this principal to prevent the lender form acquiring the collateral.