[PAULUS libro quarto quaestionum. ] §3.5.35.prSi liber homo bona fide mihi seruiens mutuam pecuniam sumpserit eamque in rem meam uerterit, qua actione id, quod in rem nostram uertit, reddere debeam, uidendum est: non enim quasi amici, sed quasi domini negotium gessit.
[PAULUS, Questions, Book IV] If a free man, serving me in good faith, has taken a loan of money and turned it to my advantage, it must be considered by what action I ought to return what was turned to our (meaning my) advantage; for he did not manage the affair as that of a friend, but as that of a master.
sed negotiorum gestorum actio danda est: quae desinit competere, si creditori eius soluta sit.
But the action for management of affairs must be granted; which ceases to be available if payment has been made to his creditor.