Allardyce is reportedly keen to bring a striker and a central midfielder to Ewood Park, having failed in his bid to sign Aruna Dindane in the January transfer window.
Chairman Williams says the Rovers boss will not be under pressure to sell if he wants to bring in new faces, with an increased television deal for all top-flight clubs on the way next season.
But he admits the final standings in the league will have a bearing on the amount available.
"It always depends on how we finish compared to what we budgeted for," he told the Lancashire Evening Telegraph.
"If we can hold our league position then we should have the Benni McCarthy money to reinvest in the squad and hopefully that should allow us to make the signings we want to make.
"We've said all along that we will be sustainable without selling anyone as long as we can hold our league position, because then we've got the new television payments coming in for the 2010/11 season.
"Every position in the league is worth around £750,000, which is massive for us.
"But drop three places and that's £2m off your budget and the Benni McCarthy money is gone."
Blackburn are currently 11th in the league table with 31 points from 26 games and they take on Bolton at home on Sunday.
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