The Ross County man – who will have a scan after going down in the match against Motherwell on Sunday – signed a pre-contract agreement with the Perth side in January and Brown intends to honour it.
The move has been in some doubt since Brittain intimated that he wished to stay with County and his injury had offered St Johnstone an opportunity to back out. However, Brown will stand by the player.
"We have acted in an honourable fashion all the way," he said. "We certainly wouldn't use an injury as a get-out. Absolutely not. That is not the way this football club operates."