[ULPIANUS libro uicensimo septimo ad edictum. ] §13.5.11.prHactenus igitur constitutum ualebit, si quod constituitur debitum sit, etiamsi nullus apparet, qui interim debeat: ut puta si ante aditam hereditatem debitoris uel capto eo ab hostibus constituat quis se soluturum: nam et Pomponius scribit ualere constitutum, quoniam debita pecunia constituta est.
[ULPIANUS, from the twenty-seventh book on the Edict.] Therefore, a promise to pay will be valid to this extent, if what is promised is a debt, even if no one appears who in the meantime owes it: for example, if someone promises that he will pay before the inheritance of the debtor is accepted, or after the debtor has been captured by the enemy. For Pomponius also writes that the promise is valid, since money that was owed has been promised.
§13.5.11.1Si quis centum aureos debens ducentos constituat, in centum tantummodo tenetur, quia ea pecunia debita est: ergo et is, qui sortem et usuras quae non debebantur constituit, tenebitur in sortem dumtaxat.
If someone owing one hundred aurei promises two hundred, he is liable for one hundred only, because that money is owed: therefore, he also who promises the principal and interest which was not owed, will be liable for the principal only.