[POMPONIUS libro sexto decimo ad Sabinum. ] §50.17.27.prNec ex praetorio nec ex sollemni iure priuatorum conuentione quicquam immutandum est, quamuis obligationum causae pactione possint immutari et ipso iure et per pacti conuenti exceptionem: quia actionum modus uel lege uel per praetorem introductus priuatorum pactionibus non infirmatur, nisi tunc, cum inchoatur actio, inter eos conuenit.
[POMPONIUS, in his sixteenth book on Sabinus.] Nothing should be altered by the agreement of private individuals, whether it derives from praetorian law or from solemn law, although the causes of obligations can be altered by agreement, both by operation of law and by the plea of an agreed pact; for the form of actions, whether introduced by law or by the praetor, is not invalidated by the agreements of private individuals, except when an agreement is made between them at the time the action is commenced.