t4e mentioned a cool way:
I grabbed the coin and dropped it while logging the cache. So the coin has his mark that it was there. Then I retrieved it from the cache, so it is in my inventory and I can drop it of when visiting the next cache. I also sent an email to the owner of the coin so he can see, what happened.
I also wrote a mail to the guys who visited the cache last. Maybe they confused the coins because the took one out and put one in. Maybe the still have the one the logged to have put in the cache.