Muffler & Brake Systems Out of town rip-off! Internet

I am originally from New Bedford and was in town visiting family when my A/C went out. I took it to Muffler & Brake to have them diagnose the problem. After about an hour plus wait they told me that the compressor was bad and needed to be replaced.

I was going to be heading back to Florida that following Tuesday, this took place on Saturday. He said he could order a new one and have it replaced on Monday. He told me that he required a deposit to buy the compressor. I spoke to my father who said I should have his mechanic check it.
 
On Sunday I took the car to my father's  mechanic and he looked at it for literally 30 seconds and said it isn't the compressor.  The new mechanic worked on the car and found a bad wire going to the compressor.  He charged $250.00 for repairs and a complete recharge of the A/C system.

Muffler & Brake were going to charge $550.00 for a new compressor. I spoke with Muffler & Brake on Monday for a refund and they refused to give me a full refund saying they worked on the car. I was charged $105.00 for a mis-diagnosis. 

I feel because we were from out of town (Florida plates) and knew we were on a schedule they felt that they could take advantage of the situation.  When confronted the mechanic said he would of noticed when he changed the compressor, but my other mechanic without any tools and in less than 30 seconds said it isn't the compressor. So I am supposed to trust that we would've been honest after changing the compressor.  And even if he was honest how much labor would've been tacked on to fix it.