a/c compressor problem

I bought a used Impreza 75,000 miles. I came out of work on a hot afternoon and started the car. When the air started to kick in, I noticed a belt squeal that put a load on the motor. It was coming from the compressor. It didn't fail but it did stop kicking out the air. I shut it off and drove home. I checked it the next day [going to work] and it worked ok. When it got hot in the afternoon, it began to act up again. Is there anything I can do to salvage this compressor? Does it sound like it need a shot of oil? Or does it just sound like it's the beginning of the end? Before I noticed this I tried to give it a can of freon [134a] but it only took half of it. I figured it was just full. thanks in advance