Isn't that what a retainer is for?

Heh, sounds like something I'd find at the corner kiosk. "I'd like to top up my pre-paid lawyer card."

I'd need a Real Attorney to chime in, but I think that, once you're actually in a trial, you can't dump your client without the permission from the judge. (You also can't intentionally stop providing your best service for any reason at all, not unless you want to be sanctioned or disbarred.) So while an attorney could insist on money up front, if the money they were paid didn't cover the real expenses, they could still end up getting stiffed. I suppose the 'smart' thing to do would be to have $100M put in an escrow account, and the money can only be released from the account if both parties agree. So if Trump tries to stiff you for the bill, he's still out the money since you don't have to release it back to him. Kinda like what The Silk Road used to do, only even shadier, since you'd be working for Trump.