This is where the Changelings should be naming their price though. If the Changelings had specific knowledge of what was being asked of them, but no knowledge that they stand to benefit from doing it, then they should be telling the players what their aid will cost. Which should probably be some kind of equivalent service in future that cannot be refused.
The fact that the players didn't use the information they had as leverage then just means they paid more than they had to, or found the price too high and walked on the deal, but at least you put a deal on the table in front of them.
And then you could ask them OOC why they didn't use their information.