Squirrel hide has been one of the historically used "standard units of measuring barter value" in Finland. The word for "money" in Finnish is "raha", and the etymology of that word derives from (squirrel) hide. This dates back to the same era UrW draws its inspiration from. So, generally speaking - foreign traders were interested in buying the valuable winter furs, and a squirrel hide was a base unit to measure the value of bigger / more rare furs. A bit simplified, but something like this =)