Yes, the contract should be honored to allow Norway to make offers on these islands; however, if the price is not correct/ fair and if the Scotts are not willing to sell it at such a price, that should be honored. Just like any transaction, both sides must agree on the product and price. If the compromise or fair value is not met, then that should not force them to sell their islands to Norway.
A 15th Century contract should not carry any weight today. The world has changed a thousand times over since then. The only reasonable way to grant the contract would be to adjust the contract to today's dollar standards. Most likely, the currencies agreed upon do not exist anymore, making it possible to exchange up to current currency at its inflated value in spite of the fixed cost.
I don't believe that the 15th century contract between Norway and Scotland should be honored today. That contract is obsolete and if honored today it would open so many controversial opportunities for other countries as well. Imagine all of a sudden different countries finding this old contracts and starting to claim their rights? I don't think that is a great idea.
This is not the 15th century and most of the laws sovereign nations follow have changed. There is no reason a contract made centuries ago should be honored especially since it was probably signed by people who we would find despicable today. Both the British and Norwegian governments should sit down, have tea and tear up the contract since it does not represent today's financial reality.