[ULPIANUS libro uicensimo sexto ad edictum. ] §12.6.1.prNunc uidendum de indebito soluto.
[ULPIANUS, on the Edict, book 26.] Now we must consider the payment of what is not due.
§12.6.1.1Et quidem si quis indebitum ignorans soluit, per hanc actionem condicere potest: sed si sciens se non debere soluit, cessat repetitio.
And indeed, if anyone pays what is not due in ignorance, he can claim it back by this action; but if he pays knowing that he does not owe, the claim for recovery is barred.