It was recently suggested in a Facebook discussion I happened to see that the city of Danzig (Gdansk) still owed money for a loan made by Karl Knutsson Bonde in 1447. This is incorrect, but the story is rather fascinating.
In the 17th and early 18th century the Gyllenstierna family considered themselves to be the closest relatives to Karl Knutsson. In 1704, as the GNW increased the demands on the Swedish economy, the matter was apparently brought to the attention of Charles XII. He viewed it as an opportunity and brought pressure on Danzig. Eventually the city agreed to pay a very considerale sum. The King made an agreement with the Gyllenstierna family that the money would be used for the war effort, but returned after the war.
When the war eventually ended in 1721 the state was very heavily in debt and the Gyllenstierna famili did not receive any money. They came back with new pleas in 1731 and 1740, but to no avail. In 1766 the issue was again brought to the attention of the authorities, but in the end the claims failed again. It was brought up again in the early 19th century, but no decision had been made when Gustav IV Adolph was overthrown in 1809. So the family came back yet again in 1823, after some more of the relevant documents had been found.
A parliamentary committee investigated the matter and came (unsurprisingly) to the conclusion that the family was not entitled to any money. The claims had come too late, it was not entirely clear that the Swedish state had borrowed the money (perhaps it was Charles XII personally) and perhaps the money had been repaid before 1709 but all relevant documents lost after Poltava?
The Gyllenstierna family made another attempt in 1834, but it went nowhere.
Some of the conclusions made in 1823 seem ridiculous. How could it possibly have been a loan made to the King personally? And how could they possibly suggest that the money had been repaid some time between 1705 and 1709? The fact that the debt was too old (it had initially been decided that war debts would have to be claimed within 20 years) was more relevant, but the family had tried both in 1731 and 1740.
So Danzig has no outstanding debt to pay (and it was of course rather peculiar that a loan from 1447 would become an issue more than 250 years later). But the Swedish army was powerful negotiating tool in 1704...
Herlitz, Nils, Från Thorn till Altranstädt (1916)
Saarinen, Hannes, Bürgerstadt und absoluter Kriegsherr (1996)
Statsutskottets betänkande 1823, nr 456 (Bihang till samtlige Riks-ståndens protocoll...4. saml., 4 bandet)