James Dick in 'The Songs of Robert Burns' took Burns' version (SMM #428) to be entirely original, but in Prof Kinsley's edition reference is given to a broadside version of c 1775 as the basis for Burns' song. [The tune in SMM is the earliest version known.] Kinsley's reference is: Ritter, Neue Quellenfunde, 1903, pp. 8-12. This I have not seen, nor can I find this early broadside on the Bodley Ballads website, or in Steve Roud's folk song index, where "Charlie is my darling", all versions, are #5510.

In Dave Harker's 'Songs from the Manuscript Collection of John Bell' (Surtees Society, 1985) is an extended version of the song in Bell's handwriting (c 1810-30?) which goes as follows:

Twas on a Monday Morning
   Right early in the year
That Charlie came to our Town
   The young Chavalier

   An Charlie he's my darling
      My darling, my darling-
   Charlie he's my darling
      My darling, my darling
   Charlie he's my darling
      The young Chavalier

All the way along the Street
   The Pipes blow'd loud and Clear
and all the folks came running out
   To greet the Chavalier

As Charlie walked up the Street
   The City for to View
He spied a pretty Maiden
   The Window looking thro!

Sae light he jumped up the Stair
   And twirled at the pin
And wha Sa ready as hersel
   To let the Laddie in

He took the maiden on his knee
   all in his Highland dress
For brawlie weel he ken'd the way
   To please a bonny lass

He kilted up her Petticoat
   and then his Phillibag
Sae Rarely and so Marrily
   They dansed the Highland Jig

He took out a Silken purse
   As long as his arm
Here take ye this fair Maiden
   I hope Ive done no harm

Its up the Misty Mountain
   An down the Foggy glen
I cannot go a Milking
   For Charlie and his Men

Kinsley notes that the last here, and in the SMM version, is from the early broadside, but is not otherwise very informative about what Burns took from it.

Note that Willing Allingham adapted the last verse for his "The Fairies" ['Oxford Book of English Verse', #776]:

Up the airy mountain,
Down the rushy glen,
We darent' go a-hunting
For fear of little men;
........

I suspect that Murray on Saltspring can add considerably to this subject.