The ברייתא of ר' נחמיה and ר' אלעזר בן יעקב

Rashi (according to Tosafos and some Rishonim) understands the מסקנה of the gemara that everyone holds like ר' יוחנן and the מחלוקת is if the מלוה did not yet collect and they argue about the nature of the איסור of לא תשימון. The ת"ק holds that it is an independent issur and therefore as soon as you make the loan you are עובר. ר' נחמיה and ר' אלעזר בן יעקב hold that the issur is only חל when the מלוה is גובה the interest. If he rips up the שטר there was no issur.

Tosafos argues that it would be strange for the gemara to make this ברייתא support ר' יוחנן as we had a ברייתא against ר' יוחנן and we answered כתנאי. Therefore they explain that everyone holds like ר' אלעזר and again the מחלוקת is what is the nature of the issur of לא תשימון . Some rishonim understand Rashi like this as well.

It comes out למסקנה that לא תשימון is not a לאו הניתק לעשה because from the fact that the לוה is over the לאו we see that it is not an ענין of חסרון and therefore giving back the money cannot be מתקן the issur. Ripping up the שטר also does not make it ניתק לעשה because tearing up the שטר is not a מצות עשה. What the Gemara means when it says מאי קום עשה לקרוע שטרא is that since there is an action that you can do, namely ripping up the שטר that will cancel the עבירה you can't get מלקות for לא תשימון because לא תשימון is only חל when the מלוה collects the interest.