[CELSUS libro sexto digestorum. ] §12.6.47.prIndebitam pecuniam per errorem promisisti: eam qui pro te fideiusserat soluit.
[CELSUS, in the sixth book of the Digests.] You promised by mistake money that was not due; the person who had become surety for you paid it.
ego existimo, si nomine tuo soluerit fideiussor, te fideiussori, stipulatorem tibi obligatum fore: nec exspectandum est, ut ratum habeas, quoniam potes uideri id ipsum mandasse, ut tuo nomine solueretur: sin autem fideiussor suo nomine soluerit quod non debebat, ipsum a stipulatore repetere posse, quoniam indebitam iure gentium pecuniam soluit: quo minus autem consequi poterit ab eo cui soluit, a te mandati iudicio consecuturum, si modo per ignorantiam petentem exceptione non summouerit.
I think that if the surety paid it in your name, you will be bound to the surety, and the stipulator will be bound to you; and there is no need to wait for you to ratify it, since you can be deemed to have mandated this very thing, namely, that payment should be made in your name. But if the surety paid in his own name what he did not owe, he himself can recover it from the stipulator, since he paid money not due under the law of nations; however, as to what he is less able to obtain from the person to whom he paid, he will obtain it from you by the action of mandate, provided that he did not, out of ignorance, fail to repel the claimant with an exception.