Millwall claim to have already matched West Bromwich Albion's demands for Chris Wood on two occasions and will not be increasing their offer.
The Lions are understandably keen to bring the striker on board full-time, with the New Zealand international in rich goalscoring form during a three-month loan stint at The Den.
West Brom are prepared to discuss a deal, with Wood some way down their attacking pecking order, but they are aware that a bidding war could be sparked in January.
Millwall claim they are being messed around in their attempts to put a transfer in place before the window opens, and say their latest bid can be considered a 'take-it-or-leave-it' offer by the Baggies.
"They told us ?500,000 and I said fine. They said ?1million and I said fine. So I've gone for it already - I'm done. That's it," said Lions chairman John Berylson.
"For us, it's a take-it-or-leave-it offer. We're not going to increase it but I'm sure West Brom are thinking about an auction.
"If we take our bid off the table, which is worth well over ?1million - that's not including a sell-on clause either - and West Brom end up selling him for half that in the future, they will look stupid. That's the biggest danger really, looking stupid.
"I know Huddersfield are in there but Chris has a flat in London."
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The 27-year-old centre-back, who can also operate as a midfielder, was released by the Millers this summer following an injury-ravaged season in south Yorkshire.
He joins North End on the back of a successful trial period and previously played under boss Graham Westley at Stevenage in the 2010/11 season.
Foster will make his debut in Tuesday night's Johnstone's Paint Trophy Northern Area quarter-final at Bury after being named in the starting line-up in place of the suspended Chris Robertson.