köken: Univerbation of ἐφ' ὅσον (ef' óson), a learned borrowing from Koine Greek ἐφ' ὅσον (eph' hóson, “for as long as possible”), with semantic loan from English as long as and/or semantic loan from German solange, sofern.
Εφόσον έχουν καταβληθεί τα πρόστιμα, είναι ελεύθεροι να φύγουν.
Compare English provided (that) for semantics, as pourvu is the past participle of pourvoir (“to provide”). The phonetic resemblance to Italian purché is coincidental.