Digestorum libri
Ex libro XC
Ad Dig. 16,2,2ROHGE, Bd. 8 (1873), S. 43: Zulässigkeit der Compensation von Gegenforderungen, obschon über letztere bereits quittirt ist, sofern die Quittung die Art der Tilgung nicht ergibt und behauptet wird, daß dieselbe nicht durch Zahlung, sondern durch Aufrechnung geschehen ist.Julianus, Digest, Book XC. Any one can bar his creditor, who is also his debtor, when he brings an action against him if he is prepared to set off his claim.
Ad Dig. 46,1,18Windscheid: Lehrbuch des Pandektenrechts, 7. Aufl. 1891, Bd. II, § 412, Note 17.The Same, Digest, Book XC. He who delegates his debtor is understood to pay as much money as is due to him; and therefore, if a surety delegates his debtor, even though he may not be solvent, an action on mandate can immediately be brought.