Whether it's safe to do depends on whather your system has any leaks. If the system has a leak and, thus, has been exposed to outside air, then, besides the obvious need to repair the leak, you really need to "boil it out" by pulling a vacuum on it and getting any moisture out of the system. If you don't, you risk your compressor siezing. Also, sticking R-134a in there without the correct amount of the proper refrigerant oil can lead to siezure.

At the very least, go through the recommended procedures for diagnosing your fault condition. Subscribing to Alldatadiy.com is one way to find the procedure; buying the Volvo manual is another.

At the worst, take it to a reputable shop and have them give you two estimates:
1. how much will it cost to make it work correctly
2. how much will it cost to get through the end of the summer

-cj