Yeah, it might not be a good idea to put the computer in that he has because it's very unlikely that he has one for a 96 and it will not start. Check yours to see if it's a reman. I just looked on rockauto.com and all they have are A-1 Cardone reman's for $136 with a $70 core. I wasn't even paying attention to you having a 96 when I suggested putting in the one he says he took out.
Honestly, I think with it being a holiday, you're car was never even messed with. Like Jamie said, it was most likely put on the back burner with him knowing that he would come back and clean your terminals and write up some high priced bill. I would find out what "catch codes" were used for 96 Marks and see if his matches. If his does match, then put it in and see if it starts. If it does not match, then you know he lied and what he has in his hand will never work anyway. What are the chances of him knowing that he has to have a specific year computer for your car?
I could be way off but I wouldn't think that no more than 2, maybe 3 catch codes were used for 96 and with it being year specific, the number may even be dropped to one, maybe 2. I would find out what these numbers are and if his do not match, do not bother pulling yours to put his in. It's going to fail if it don't match and he's going to say "SEE"
He also could know quite a bit about Mark VIII's and thinks that you don't. If that's the case, prove his ass wrong. The one main thing you have going for you here is that you have a 96 and I would use that to my advantage. He could be doing the same thing or he could be Mark VIII stupid and if you're loaded with info written down, you'll drive away, letting him know that the BBB will be called.