Ford Repair: air conditioning, ford lincoln, lincoln navigator

Question
First of all, thanks for any help you can provide!!

We had our SUV  (2001 Lincoln Navigator) in the shop over the last 2 years replacing the heater core 2-3 times.
This January '07 they replaced the radiator - it was leaking.  The first time we tried the A/C this year when it was hot, the A/C did not work.
We took it in expecting the shop that did the work to cover any problem.
Boy were we wrong!  They are telling us that the compressor seal is bad and we will need a new one ($700 total expense).
I feel that it is possible that it was something they might have done during the previous visits to the shop.
The service manager is telling me that there is no way they could have done anything wrong.
What are the possible causes of a compressor leaking at the seal?  If they forgot to charge the system up or could overcharging cause this?

Thanks again, Steve

Answer
well sorry to hear that .... but the a/c sysem  is very weird . it may work today and it may not later that day , i would ask them if they can just replace th seal , which is possible on most vech, also overcharging the uint may cause it , front seals on compressor is a very common leak on ford/lincoln