I may start with one i found some time ago, but since game hasnt been updated after it, this should work just fine.
I am not 100% sure about needed relation level, it has worked for example at +87 for me, may work lower as well, but mostly i have played it safe, and done it with +100 relations. I have noticed that AI gets angry after some time, but it needs many trades to get mad. Or it may get mad for other reasons, i dont know. And when i say it gets "mad" it just lowers the relations with you, via insult.
OK, but heres how it works:
-You have good relations with any AI country, rebels with no money work as well.
-Then you ask for trade deal with them, you will ask 100 000 money from them, because it is max limit at the r´trade deal.
-You offer them no attack pact, idk how it is called, i am Finnish.
-Then you just send trade request, and they will accept it always, if you have good enough relations with them.
-Do not ask for trade with the same AI country every turn, you must keep at least 1 turn between trades for this to work.
-If your relations are low, make sure to send diplomat before asking for trade, in fact best case would be if you would always have diplomat improving relations.
-It doesnt matter whether or not AI controlled country actually has money, i guess it just borrows it and ruins its economy forever, never checked.