Grammaticalization or loan translation?
The Anglo-Norman phrase pourvue est que (rendering Latin provisum est quod) was grammaticalized in the mid-14th century as a specialized participle-conjunction phrase pourvue que, introducing clauses which expressed the sufficient and necessary condition. At the turn of the 15th century bilingual London Chancery writers copied the conjunction into their Middle English legalese as purveyed that, later replaced with provided/providing that. The process was parallel with the gradual elimination of the verb purvey(en) in favour of provide(n). Around 1600 that became optional and the use of the phrase was extended to other genres, thus completing the grammaticalization of the new conjunction in English.