The Comprehensive Aramaic Lexicon

Trf 2.3.107 :wmdy gw mn dnr ḥyb mksʾ hyk ʿdtʾ ʿ[r]pn yhʾ gbʾ : whatever has tax due of less than a dinar by custom shall be collected in change

See the entire entry for ˁurpān, ˁurpānā n.m. coinage, small change