The keeper, on a season-long deal at The Hawthorns, admits he will almost certainly go back to his parent club if they go up, and attributes their League Cup win for their demise
The former Manchester United shot-stopper moved to The Hawthorns in the summer on a season-long loan deal after Blues were relegated last term, and he admitted after helping the Baggies secure a draw against Fulham that he would almost certainly not be staying if Chris Hughton's side secure promotion.
"As far as it stands it was a season-loan deal and that was that,” he told reporters.
"There was no kind of talks about if Blues stay up or if they get promoted or if they go down.
"There was nothing like that and no other clauses in the contract.
"It depends on what happens at the end of the season, if Birmingham go and get promoted and get back in the Premier League then I will be back there playing football with Birmingham - simple as that.
"If it doesn't happen then there is a decision to be made on both parties.
"I will have to bide my time and wait and see."
Despite West Brom's poor start to the season, the goalkeeper was confident that they would not suffer the same fate as Birmingham who went from the high of a League Cup win to slipping down to the Championship last season.
Indeed, Foster attributes the 2-1 win over Arsenal at Wembley as the main reason why Blues eventually fell into the bottom three whilst his current employers were able to spark a revival under Roy Hodgson and climb up the table to finish in 11th place.
"I think it was the cup that really did it and what took us down,” he added.
"It took so much out of us, more emotionally than anything. It was so tiring.
"I remember literally for two weeks afterwards the lads in training were just knackered.
"It was such a big effort from everyone it really did affect our Premier League form.
"There were some games towards the end of the season, particularly when we lost 2-0 to Fulham, it was such a poor performance from the lads it seemed like we were almost resigned to the fact we [would be] relegated.
"You could see something like that happening which was disappointing.”
West Brom were knocked out of the competition when they lost 2-1 in extra-time to Everton last week, but despite his belief that the cup was a distraction for his parent club last season, he revealed he was disappointed with the Baggies' exit.
"Well I would love to get to the final again,” he said, highlighting the conundrum which Premier League managers face when trying to select their team for cup matches.
Foster insisted that he had seen enough in the West Brom squad to know that a relegation battle is not on the cards.
"We'll be absolutely fine we will,” he said.
"We had the misfortune of getting relegated with Birmingham last season and I had the misfortune of getting relegated with Watford as well.
"I can see what you need in your team to avoid the drop. We have plenty of talent in this team to avoid the drop.
"I think realistically we have to be looking towards 10th and 11th places. That's our true position.
"We have just had a few disappointing results to the start of the season with the teams we have played against.
"I would like to think we will start picking up some points soon this season and start climbing up the table.”
One area that has been of particular concern to Baggies’ fans is the lack of clean sheets which has made it difficult for West Brom to stay in the Premier League for more than one season.
Since getting promoted in 2002, they have spent three seasons in the Championship, with this being their sixth in the top flight.
In 2009 they had the worst defensive record when they finished bottom and last year only Blackpool had a worse record.
Saturday’s result, however, meant they already have their second clean sheet this season – matching their efforts for the whole of last season – and Foster expects to see many more as the year progresses.
"I would like to think [we will get more clean sheets],” he said.
"We played six games this season and kept two clean sheets this season, it should have been three because of the Stoke game.
"I feel pretty comfortable with the team we have got and the back four especially.
"We'll keep a few clean sheets this season.
"At Birmingham we kept nine or 10 last season and I would like to keep a similar kind of tally this season as well.”
West Brom will continue to keep clean sheets if Foster makes saves like the one which prevented Bryan Ruiz from scoring on Saturday, where he diverted the striker’s close range shot onto the post.
However, the 28-year-old admitted that he had not realised he had made the save, before adding the players were disappointed not to have secured the team’s first home win of the season.
"I didn't even know I touched it,” he said.
"I don't know to be honest, I didn't think I got a touch on it.
"I was quite surprised he managed to put it wide from there. I was surprised he managed to miss the rebound.
"Considering it was 0-0 it was a fairly decent game.
"First half was probably a bit disappointing for the fans but second half I thought we were the better team.
"We had a lot more possession and created a good few chances.
"At the end of the day we are probably a little disappointed we didn't get three points.”