[ULPIANUS libro tertio disputationum. ] §44.2.23.prSi in iudicio actum sit usuraeque solae petitae sint, non est uerendum, ne noceat rei iudicatae exceptio circa sortis petitionem: quia enim non competit, nec opposita nocet.
[ULPIAN, on Disputations, book three.] If an action has been brought in trial and interest alone has been claimed, there is no need to fear that the exception of res iudicata will cause prejudice regarding the claim for the principal; for since it does not apply, it does not cause prejudice even if raised.
eadem erunt et si quis ex bonae fidei iudicio uelit usuras tantum persequi: nam nihilo minus futuri temporis cedunt usurae: quamdiu enim manet contractus bonae fidei, current usurae.
The same will hold true even if someone wishes to claim only interest on the basis of a good faith action; for nevertheless, interest for future time accrues, because as long as the good faith contract remains in force, interest will run.