Speaking as a Dom - and we do have differences of approach, cadence - I'd agree with gem, amber and blizz - there are RED FLAGS all over the place. The fellow is far too insistent. For starters, he knows you're collared elsewhere. He also knows that what you were looking to establish is a (possible) play date under rules set by your Dom. For him to push so hard, not just for a play date, but for you to leave your current Dom and join him, and to compound it by saying he would not follow the ground-rules your Dom sets for the play date are all signs that he is likely bad news.

The first two points indicate he's not particularly honourable, and the latter (not following established rules) prove he is not a Dom of his word. That means trust is something he doesn't care about at all. To me that spells predator and I'd advise you to stay away.

Talk to you Dom - tell him your concerns - and the hassles you're having. If you have to, ask your Dom to forbid you to speak with that dominant again. For me, that would be an automatic response (forbidding further contact) for my sub's safety. I'd also want that dominant's contact address and ask him firmly to back off.