From "Scotland: Story of a Nation" by Magnus Magnusson (what a name):

"King Kristian doubtless had every intention of redeeming pledges [the mortgage of Orkney & the Shetlands]; it was a meagre sum after all - in five years' time, for instance, he would borrow twenty-five thousand florins to go on a pilgrimage to Rome. By then, however, the five centuries of Norse rule in Orkney and Shetland were effectively over . . . By an act of Parliament in 1472 the islands were annexed and united to Scotland. Until the eighteenth century kings of Denmark would make recurrent attempts to open negotiations for the return of the islands, but the Scots simply refused to discuss the matter."

I just finished reading "The Story of Scotland" a week or so ago, as the referendum was approaching. I'd recommend it if you're looking for pre-Union Scottish history, as the author breezes through Union Scotland in less than 100 pages.

Edit: To clarify some figures. From 1266 on, Scotland was to pay 100 merks a year in rent for the Orkneys and Shetlands, although it was rarely paid, and wasn't paid at all after 1426. Instead of paying for the entire 10,000 florin dowry of his daughter for her wedding to James III of Scotland, King Kristian paid 2,000 florins and gave Scotland the islands until he could pay the remaining 8,000. He never did, despite as the author states, it was well within his means.