“A conditional debt if paid could be recovered as an indebitum, so long as the condition was outstanding.” W.W. Buckland, A Manual of Roman Private Law 255 (2d ed. 1939).
indebitum
indebitum (in-deb-i-t[schwa]m), n. & adj. Roman law. A debt that in fact is not owed. • Money paid for a nonexistent debt could be recovered by the action condictio indebiti. Cf. DEBITUM.