I have a vague notion acquired somewhere that an American president can be sued, but the action is delayed until he leaves office. Could be a dream on my part.

But sure the hell would be interesting. Surely the reputation of a former president tarnished internationally by a sitting president with full daily coverage for weeks is worth a couple of hundred million....very nice contribution to a presidential library. The amount would have to be HUGE to get Trump's attention. 

Oh what fun that would be...better court drama than OJ!