Till thou have paid (heōs apodōis). Second aorist active subjunctive of apodidōmi, to pay back in full.
The last mite (to eschaton lepton). From lepō, to peel off the bark. Very small brass coin, one-eighth of an ounce. In the N.T. only here and Luk 21:2; Mar 12:42 (the poor widow’s mite) which see note.