Louis van Gaal says Manchester United have offered David de Gea "a lot of money" in a bid to keep him at Old Trafford.
The 19-year-old midfielder, son of Sunderland manager Gus, is a graduate of the Charlton academy and was voted Player of the Year last season despite only making his debut in January.
He turned down the offer of a new contract with the Championship side earlier this summer amid widespread interest in his services.
Charlton will be due compensation for Poyet as he is the under age of 24, with a tribunal settling the fee.
West Ham have already signed Mauro Zarate, Cheikhou Kouyate and Aaron Cresswell this summer.