This is definitely something that there's no point giving the AI the ability to do right back at you - because you won't fall for it.
My ideas:
A third-party (something like a bank) who oversees any "credit per turn" trade. The buyer has to pass on to that third party a certain amount as confirmation of the trade deal (say 10-20% of the total bc to be paid). Then, that third party pays out to the race you've bought from the credits as per the agreement (say 7bc/turn). Then, after the third party has used the deposit you've given them, you have to stump up more/the rest of what you owe (if you originally passed on 10% either a further 10% or the remaining 90%). And if you renege, well, suddenly all that stuff isn't yours anymore. Title in the property doesn't transfer until full payment is made.So you might not want to move those ships too close to your best planets until the deal's done and they are yours.
If the buyer declares war before full payment, the deal is cancelled. Planets/ships are once again property of the seller. And will attack you if possible.
If the seller declares war before full payment, the deal is cancelled. Buyer keeps all monies (even those paid so far in the credit/turn deal) and, of course, all planets/ships etc bought in the deal.
