"autolycus"; The short answer is, "it depends..."

Burns wrote new words to existing airs; sometimes these airs already had words, sometimes they were simply instrumental, sometimes it was half-way between the two: that is, the earlier words might have been entirely, or partly, forgotten, "buried midst the wreck of things which were". David Herd had collected many fragments, as well as the completer songs and ballads which he published in 1769 and 1776; Burns made use of some of these fragments in his own compositions in a traditional idiom. Sometimes, as RB put it himself, "little other than the title is ancient" (i.e., he took a hint from the title, and the spirit of the melody, in crafting his own version).

I'm not entirely sure what you meant by "provided" the tunes; on only one occasion did RB compose a melody himself (he mentions it in his Letters, somewhere!), but on others - as you may read in my conrtribution of February 7th, above - he evidently DID "provide" a set of the melody to publishers. Most frequently, however, he would simply indicate in which published collection an instrumental set of the melody could be found. Not that they always heeded him; GT, Gods help us, even published "Wert Thou in the Cauld Blast" to "My Love she's but a Lassie yet".