But if it's a forgery, it's not yours to begin with, so why would there be a need for permission. It's just propaganda.
There is a significant difference.
If the post said explicitly that the letter was forged, then it would probably be acceptable (but still borderline).
Otherwise, it is just my word against his that it is or isn't a forgery. Therefore, it should never have been allowed to be posted without my permission. I'm sure this has been discussed elsewhere about the full transcription of in-game messages to out-of-game locations. This should NOT be allowed.