How on earth can this happen?
The ESCROW service means that the buyer does not get title to the aircraft until all the money is where it is supposed to be. It sounds as if one wasn't used here which is like playing Russian Roulette with 5 out of the 6 chambers filled!
Use the purchase agreement and evidence of the payments made to get a posessory lien on the aircraft until either:
a) The remaining funds plus costs are recovered
b) The aircraft is returned on the understanding that the amount paid will be refunded minus a sensible rate for the flying done, costs, and expenses of restoring it to whatever register etc.
There WAS a purchase agreement wasnt there?!?!?!
If the aircraft is large enough to be covered by the Cape Town Convention register an interest in it through the convention double lively. When doing so hope that the new owner hasn't complied otherwise you could be in deep deep, deep cack if and when this ends up in court.
I'd also be giving Europol a call, if it was my aircraft as far as I was concerned it would have been stolen by deception.