Millwall will break their transfer record and make Tom Bradshaw their first £1m player.
The Lions have agreed a fee of £1m rising to £1.25m with add-ons for the Barnsley striker, who is expected to join the Championship side on loan on Thursday with a permanent move to go through when the window reopens in January.
Barnsley, who have made a fine start to the season after relegation to League One, were keen to keep hold of Bradshaw and rejected a number of bids.
However, Millwall eventually matched their valuation of the player.
Barnsley will now step up their interest in Portsmouth striker Conor Chaplin, 21, as they look for a replacement for Bradshaw, who joined from Walsall for around £600,000 in 2016 and who scored 18 league goals for the club.
Millwall's existing record transfer is the £800,000 they paid to Derby County for Paul Goddard in 1989.
