According to Bild, the two clubs are now discussing a fee in the region of £30m — well below the €50-60m Mainz had been demanding just days earlier, and a figure the Bundesliga club had previously ruled out entertaining this late in the window.
Sporting director Christian Heidel had said as recently as last week that Mainz would not sell on deadline day itself, insisting the club needed enough time to find a replacement before sanctioning any departure.
The proposed structure appears to be softening that resistance. Sano's offer is understood to include a substantial sell-on clause that would give Mainz a share of any future transfer, a mechanism that could make a lower initial fee more palatable given how central the 25-year-old has become to the club's midfield.
Sano is regarded as Mainz's "Iron Man," having logged close to maximum minutes across all competitions since arriving from Kashima Antlers for just €2.5m two years ago — a fee dwarfed by his current market valuation of around €50m.
The player himself is said to be keen on the move, with sources in England suggesting Sano "has no better market elsewhere for various reasons," an indication that Crystal Palace may be seen as his most realistic route into the Premier League this window.
The south London club have endured a difficult start under new head coach Pierre Sage, losing their opening two league fixtures, and had been searching for a further midfield addition with Europa League football also on the horizon this season.
Sano, capped 16 times by Japan, still has two years remaining on his Mainz contract, meaning the Bundesliga club are under no obligation to sell if the numbers do not work in their favour.
He typically operates alongside Nadiem Amiri in central midfield, a partnership Mainz have built much of their defensive structure and build-up play around this season. With hours remaining before Tuesday's deadline, the decision now rests with Heidel and his recruitment staff over whether to accept a fee well short of their original valuation or keep hold of a player who has become fundamental to how Mainz build attacks from midfield.