"Child C: toucher or gifts to Barbara Allen- present in Irish versions, some US versions and Buchan"

It seems to me that the tocher (dowry) and the gifts are two quite different motifs. In Child C, the issue is the man's motives for wooing Barbara. He's more interested in her dowry than in her beauty – or at least so he says, and she takes him at his word. In versions where the man leaves her gifts, these may sometimes, as apparently in Buchan's long text, be intended to supply a dowry so that she can marry someone else in the future. However this isn't always given as a reason, and the gifts often include things of symbolic rather than monetary value like bloody clothes or a basin full of his tears, which would be of no use in a dowry.