The Villans had wanted to make that move permanent themselves, but due to financial problems at the club they are not in the position to make that happen.
Foster, meanwhile, has been the subject of a rejected £1million bid from the Hornets, who are expected to come back with an improved offer for the Baggies stopper.
Competition for Johnstone
The 25-year-old United 'keeper has already shown he can shine in the second tier of English football and therefore should Swansea City consider a move for the former England Under-20 international?
Like West Brom, the Swans have just been relegated from the Premier League and could also be in the market for a new option between the posts following the sale of Lukasz Fabianski to West Ham United.
At £5million, Johnstone represents a reasonably cheap option considering his age and experience and could be just what new manager Graham Potter needs following the departure of influential and popular Fabianski.