If transmission fluid was put in a master cylinder.

This was put in by an incompetent person at a repair station. We asked the company to reimburse for damage since they did the brake job, but they said it was to long after and the fluid would have damaged the rubber gaskets a lot sooner. This was 5 months after their brake job. The person who repaired and replaced the master cylinder said it it could take up to six months for the rubber to fail. What do you think?

That's possible but as stated there is no way to know for sure, plus I guess it depends on how much was added. It might not hurt a thing... Just to be on the safe side, I would have the fluid changed!