[GAIUS libro singulari de casibus. ] §12.6.63.prNeratius casum refert, ut quis id quod soluerit repetere non possit, quasi debitum dederit, nec tamen liberetur: uelut si is, qui cum certum hominem deberet, statuliberum dederit: nam ideo eum non liberari, quod non in plenum stipulatoris hominem fecerit, nec tamen repetere eum posse, quod debitum dederit.
[GAIUS, in his single book on Special Cases.] Neratius reports a case where a person cannot recover what he has paid, as if he had given what was due, and yet is not released from his obligation: for example, if someone who owed a specific slave has given a conditionally manumitted slave. For he is not released for this reason, namely that he did not make the slave fully the property of the stipulator, and yet he cannot recover the slave because he gave what was due.