I work on the P9S probably more than anyone, so I'll be glad to help.
There is not easy way to remove the recoil buffer. Any method of prying it out with a pick or similar tool will most likely destroy it. With such a classic gun, long out of production, with parts difficult and expensive, to impossible to source, I always recommend new owners go ahead and just replace the recoil buffer unless you can see from the limited view that it looks brand new. Most of the time though, the buffer has not been changed since HK originally put it in (30-40 or more years ago). Even if the pistol was a safe queen, chances are that the buffer has degraded over time to the point that it will not protect the rather delicate locking lug area of the receiver.
If you never need any support with your P9S or any other HK weapon, give me a shout.