Retiring/Old player

I have a 33 yr old player who is not retiring this season but I'm sure he will in the next season. He would "tell" me that he is retiring on the 1st day of the season and I would not be able to sell him (only sack him or keep him) as soon as the maintenance is done on the 1st day of the season, correct?

Yes. That would be correct, I had a GK I bought last season that done the same thing and decided to keep him on this season as a high-end 4* reserve since he wouldn't be playing too many games.