Blown transmission

I have a 1986 Mercedes 230 with 95,000 miles. I just brought it in for service and they changed the engine oil and flushed the transmission. I drove the car immediately home which is less than 5 miles. The next day my wife went out to do errands and within a mile bad noises came from the rear end. She stopped the car and fearing the worst she had it towed back to the shop. As it turns out-so far, the transmission is shot and the mechanic said that he found pieces of metal-DAAH! He claims that he put the oil back in, I have no proof. I believe that it's just a little ironic that this happened within minutes of getting the car back. He wants to charge me saying that I should have had the transmission checked before. It doesn't make any sense. I need the car he is holding it and I need to know what rights I have and how to handle this problem.
Thanks, David
Savannah, Ga.