The concept of זה נהנה וזה לא חסר which is considered midas s'dom so we force the person who has nothing to lose to forgo his claim, seems to be d'oraysa. Although we don't really have explicit pesukim in the Torah forbidding specific bad middos, we can prove from Tosafos that it is assumed to be d'oraysa. Tosafos asks why we need a pasuk to say that a b'chor is entitled to both his portions next to each other, rather than dispersed among the fields of the other brothers - even without a pasuk we should give the b'chor to adjacent portions based on כופין על מדת סדום. From the fact that Tosafos assumes that this concept would render a pasuk irrelevant, it is clear that Tosafos assumes that it is a d'oraysa concept. Even according to the second answer of Tosafos in the name of Ritzvah that: הא דאמר רבה כופין, לא מדין תורה קאמר דבדין היה יכול למחות שכנגדו. The Ritzvah is not undermining the entire concept of כופין על מדת סדום from being a d'oraysa. Rather he is saying that since the direction of the field does make a difference, it is not truly a case of זה נהנה וזה לא חסר, and therefore using the midas s'dom argument you cannot force the other brothers to provide the b'chor both portions next to each other. Only because we have a pasuk that entitles the b'chor to both portions next to each other can he actually demand that. But surely the Ritzvah would agree that when it is in fact a case of זה נהנה וזה לא חסר, it would be m'doraysa to insist that the one who has nothing to lose forgo.