My question involves a consumer law issue in the State of: MI

I was contacted by a person that wanted to buy a computer from a person that I had worked with. Lets call the seller mr.x and the buyer mr.z. Mr.z wanted me to transfer money from him to Mr.x through paypal since Mr.x didn't have paypal. After making sure that they both knew and understood that I was only responsable for transfering the money I agreed to the deal and did just that, I accepted 600.00 electronically and gave it to Mr.x, there was a mixup and Mr.x filed a claim getting his moneyback. I told Mr.z that I had done my part and that I would sue if he did not make his part right. A few days later I was contacted by Mr.z and asked if we could settle. I was asked if I was wired half up front if I would go to Mr.X and make sure he had the computer, then go with him to the post office and send the confirmation number to him, at which time I would reciever the second portion of what Mr.z owed me. I went to Mr.x and took him there, and sent the shipping info to Mr.z, as I was asked to do.
Now the package has arrived and Mr.z wants to claim that all he got was paper in a box. I have no idea exactally what was sent, I saw a computer boxed up at Mr.x's house and etc but I can not guarantee that he did not have an opertunity to switch boxes etc, but I also have no proof that Mr.z didn't recieve the computer and is now saying foul because he wants the computer and the money also. I also have emails that from the start he was screaming scam etc and talking stuff to that status. Now he claims that I need to basically go beat a computer out of the guy or give him money that I was out for doing the original transfer or he will file in court.
My question is since I did my part, taking him to the post office and etc does he have a case against me or just Mr.x?