Millwall head coach Alex Neil says the club will receive "significant money" if Romain Esse moves to Crystal Palace during this transfer window.
The 19-year-old signed a new "long-term" contract at The Den in the summer but has been linked with a January move to Selhurst Park.
Neil confirmed the Premier League club's interest in signing the winger this month.
A source told BBC Sport that Esse's new contract includes a release clause which would need to be met for the attacker to leave the club this month.
Speaking ahead of Millwall's FA Cup third round tie against Dagenham & Redbridge on Monday, Neil said the club wanted to keep hold of their best players.