This Gaelic proverb may have been sourced, by Dorothy Dunnett, from an earlier literary source. She doesn't say, neither does the author of her concordance. So I am looking around for something earlier. And I found:

Fóghnaidh salann salach air ìm ròinneagach.
Sufficient is dirty salt for butter covered in hair
/Dirty salt will do for hairy butter.
page 186,
A Collection of Gaelic Proverbs and Familiar Phrases,
Based on MacIntosh's Collection
edited by Alexander Nicolson
Edinburgh: MacLachlan & Stewart, 1882