Not really an AIBU but how do you go about getting money off people who manage to get a considerable sum from you quite stealthily, by saying "oh, if you get/book/buy mine, and I'll give you the cash later?"
But "later" never comes 
I've now realised this person has form for this but I'm still considerably out of pocket and I want to claim mine back - any ideas for how I can politely but firmly demand it back? :)