These scenarios, even where legit, have the potential to go very wrong very quickly - as you observed in your post, all it takes is for him to level accusations of theft, blackmail or fraud and you could be in for some seriously unwanted attention (plus various other complications if transactions come into question, or if they're married and the money isn't entirely 'theirs')
Possibly the 'safest' (and I use the term loosely) mechanism is to work the arrangement through a joint account and that way there's less chance of such accusations but not by much; it doesn't deal with questions of vulnerability and still leaves you open to FS scrutiny.
As with any fetish based arrangement, the important thing will be to have clear, mutually agreed parameters in writing (or otherwise documented) beforehand - a very basic findom agreement could be that you can demand cash payments at any time up to a value of x per calendar month, or that he purchase items specified by you up to a value of x per month, or he gives you responsibility for x amount with minimum allowance to him of y per calendar month (i.e. amount minus your fee).
Personally, I wouldn't go anywhere near anything like this, but it's workable if you're prepared to take the risk and have plenty of precautions in place.