The version I found was called The Seal Killer (or was it the Seal Hunter...I don't want to turn on the Nook to check). A man was supporting his family by killing seals and selling their fur. One day he tried to kill one but it got away with his knife still stuck in its flank. The man later has a visitor insisting he come because his laird wanted a bunch of skins pronto. He's put on a horse with this strange rider (I think its a waterhorse, but its not described) and is carried extremely fast to the sea. They dismount and the rider pulls himself and the seal hunter into the sea, changing them both to seals on the way down from the cliff, using his breath. While down there, he's shown the knife, which he claims as his. He's then asked to heal the injured seal (who is the rider's father) by a laying on of hands sort of thing. Then the man has to promise to never kill seals again. He's rewarded well enough not to ever need to go back to his old ways, but it's implied that if he goes back on his word something bad will happen.
I found this in the collection of European Fairytales that is available on Nook and Kindle, in the Scottish section.