[ULPIANUS libro quadragensimo tertio ad Sabinum. ] §46.3.1.prQuotiens quis debitor ex pluribus causis unum debitum soluit, est in arbitrio soluentis dicere, quod potius debitum uoluerit solutum, et quod dixerit, id erit solutum: possumus enim certam legem dicere ei quod soluimus.
[ULPIANUS, in the forty-third book on Sabinus.] Whenever a debtor makes a payment on one debt arising from several causes, it is at the discretion of the payer to state which debt he prefers to be paid, and what he has stated will be considered paid; for we are permitted to impose a certain condition on what we pay.
quotiens uero non dicimus, in quod solutum sit, in arbitrio est accipientis, cui potius debito acceptum ferat, dummodo in id constituat solutum, in quod ipse, si deberet, esset soluturus quoque debito se exoneraturus esset, si deberet, id est in id debitum, quod non est in controuersia, aut in illud, quod pro alio quis fideiusserat, aut cuius dies nondum uenerat: aequissimum enim uisum est creditorem ita agere rem debitoris, ut suam ageret.
But when we do not state towards what it has been paid, it is at the discretion of the receiver to which debt he credits it, provided that he determines it to be paid towards that which he himself, if he owed it, would have paid and from which debt he would have discharged himself if he owed it, that is, towards that debt which is not in dispute, or towards that which someone had guaranteed for another, or of which the due date had not yet arrived; for it seemed most equitable that a creditor should conduct the affairs of his debtor as he would his own.
permittitur ergo creditor constituere, in quod uelit solutum, dummodo sic constituamus, ut in re sua constitueret, sed constituere in re praesenti, hoc est statim atque solutum est:
Therefore, the creditor is permitted to determine towards which debt he wishes the payment to be applied, provided that we determine it in such a way as he would determine in his own affair, but he must determine it on the spot, that is, immediately when the payment is made: