Milligan has been the subject of transfer speculation throughout the A-League off-season with the 27-year-old defensive midfielder having been linked with Baniyas of the United Arab Emirates, plus Queen's Park Rangers and Crystal Palace.
The latter, who will play in their first English Premier League campaign since 2004/05 in the upcoming season, have reportedly made a £1million bid for Milligan, but having sold Billy Celeski to Al Shaab on Tuesday, Postecoglou is not planning to further weaken his midfield stocks.
Postecoglou has also warned Palace that Victory are not playing hard ball simply to earn more money from the sale of Milligan.
"We haven't received an official offer for him and, to be honest, if we did we're not going to let him go," Postecoglou told the Herald Sun.
"He's contracted for another two years, he's a very valuable member of our football club and that's the reason why we brought him here.
"We don't really need the transfer money, we're not looking to make money.
"Ultimately our intention is, from a football perspective, to keep him at the football club."
Postecoglou confirmed Victory will receive an undisclosed transfer fee for Celeski, while adding the 28-year-old left Melbourne on good terms.
"He's got a chance to experience something new, he's never played abroad, and he's also got a chance to get some meaningful financial gain," Postecoglou said.
"He had a clause in his contract that the overseas club has met so he leaves with our blessing."
Milligan played 22 matches for Victory last season, winning the club's player of the year award, and his form in the A-League saw him break into the Socceroos' starting 11 during Australia's final three World Cup qualifiers for Brazil 2014.
With the World Cup finals looming, Postecoglou believes Milligan is poised for an even better season.
"People forget that he came in pretty late in the piece last year and then at the start of the season we had him at centre back but we know where his best position is and I think this will be a massive year for him," Postecoglou said.
"Touch wood he stays healthy and if he does then he'll be right amongst, not just the World Cup squad but actually playing in Brazil.
"That's what he wants and we certainly want that for him."
