I say we agree to their terms, get the proof, and execute him anyway. A post-execution exchange might go something like this:
Justice Minister Herta Daeubler-Gmelin: But, you said you vould not execute ze prisoner!
GWB: Well, you know, we didn't mean to kill him, Herta, but a funny thing happened on the way to the delousing chamber.
I say we contract the Mossad. Get them to steal the evidence for us and as a thank you we give Israel a nice shiney piece of military hardware.
oops, didn't mean to kill the bastard. all we have to do is say we're sorry... won't do it again.