Sorry, but there's really nothing you can do about it and you can't expect niko to force leo to do the trade. You can expect someone to uphold a deal they agreed to, but in all reality, they can back out at any time if they got a better deal. Leo has been working angles non-stop with everyone. He also offered me a deal and then backed out while trying to muscle me into a different deal that I never agreed to. I'm not going to complain about it, though. I'm not even mad about it. It's the way dealing goes down sometimes.
Bottom line is this, you're going to have to suck it up and move on. He's not breaking rules and there's no enforcable code of ethics. What's being a shrewd GM to one person is just being a dick to another person. If it turns out that more people end up feeling the later about leo, he'll be unable to deal in this league and will have a rough go of it.