The money is what makes it tricky - otherwise - assuming he's not a maniac - the stop word will work and you can terminate the session. In fact, that's how a safe word is supposed to work. There's usually a caution word (e.g. "Yellow") before you get to STOP word (e.g. "Red"). And the STOP word is supposed to mean STOP. In other words, session over! You get untied - DONE. In a civvy type thing - that would (or SHOULD) really be it for the evening.
You really do need to wind it up rather than continue because the dynamic is just going to get worse - you two are just not compatible. Some subs are into masochism and abuse - you aren't. You are apparently willing to do sensual submission. You are willing to let him tease you and restrain you as long as he does it in a way that you find acceptable - or basically on your terms. You want to top from the bottom - that's perfectly OK - I occasionally indulge ladies who are into that...
BUT that apparently isn't what he's into... Unfortunately, he's apparently into control and inflicting pain is a way of gaining control. The knots are a dead give away.... they are tied that way for a purpose.... You just aren't going to get along. You are trying to top from the bottom - and he's not interested - especially if he REALLY is an alpha dom. In fact, he might take the "top from the bottom" thing as a deliberate challenge. In other words - from your description, the two of you have a very different expectation of the session. You are assuming incompetence - and that may not be accurate. He may have made an assumption about the session which was not accurate.
On top of that problem - since there is money involved AND you want to keep all that money - It's just not going to be a good situation. I can't tell what's appropriate - but I can predict that the outcome is not going to be mutually satisfactory...
In order to avoid the situation to begin with - you really need to get to know somebody over a session or two before you let them tie you up and have their way with you - especially since you really don't seem to be a pro-sub and from your description, you really aren't all that submissive to begin with. Further, next time, (if there is one), let the guy know upfront that you are not a masochist and what you are up for is some gentle teasing and service submission rather than full bore S&M. Go over the toy protocols with him since that is important to you.
That said, you might want to really rethink offering this as a service to begin with....
BTW as a suggestion - you ought to be using your toys that only have been used on you.. That way, you won't need to worry about HIS competence and diligence in putting on the covers and keeping YOU safe....