Not saying this is what actually happens in the LN (I'm caught up on the official LN release), but if you think about it, the salt mine deal is perfect. Right now they can't do anything with it. It's like loaning your neighbor a lawnmower you got for free when you moved in anyway. A lawnmower that you know your neighbor was already sneaking over during the day while you were at work and using without your permission. You're out nothing, your neighbor is happy and buys a little loyalty, and if you need it back later you take it back. It puts you one up on your neighbor for almost zero cost, that douche is now paying for the gas he was originally stealing, and who doesn't want to be one up on their neighbor. Damn sneaky, nosyass neighbors.