[POMPONIUS libro uicensimo septimo ad Sabinum. ] §12.1.3.prCum quid mutuum dederimus, etsi non cauimus, ut aeque bonum nobis redderetur, non licet debitori deteriorem rem, quae ex eodem genere sit, reddere, ueluti uinum nouum pro uetere: nam in contrahendo quod agitur pro cauto habendum est, id autem agi intellegitur, ut eiusdem generis et eadem bonitate soluatur, qua datum sit.
[POMPONIUS, On Sabinus, book 27] When we have given something as a loan for consumption, even if we have not expressly provided that an equally good thing should be returned to us, it is not permitted for the debtor to return an inferior thing, even if it is of the same kind, as for instance, new wine instead of old; for in contracting, what is intended must be held as if it had been expressly provided, and it is understood that this is intended, namely, that performance should be made with the same kind and the same quality as that which was given.