1999 Dodge Intrepid A/C

Air Conditioning problem
1999 Dodge Intrepid Front Wheel Drive 160000 miles

I took my car into a mechanic to have the a/c fixed as it was not blowing out cool air. He said I needed an expansion valve so I paid him around $500.00 to fix the a/c. A month later the a/c starts blowing out luke warm air so I take it back to the mechanic who had fixed it now he's telling there's a leak that's why its not working. He wants $400.00 to fix it again. What type of guarantee's do mechanics have when it comes to repairing a/c in a car and should the leak have been detected the 1st time he looked at it? Cause he said it wasn't leaking then, could he not have seen that the hose or whatever it is was worn out?
it feels like quite a rip off