Propper, 27, who became Albion's record signing when he arrived from PSV Eindhoven for £10million in the summer of 2017, is valued at £18millon by the cash-rich Dutch giants.
The Dutch international has been a key performer for Chris Hughton as Brighton have reached the FA Cup semi-finals.
Ajax sporting director Marc Overmars is a big admirer of Propper and has failed with a bid to sign him before.
Propper is aware of Ajax's interest but refused to talk about a return to Holland when he was away on international duty.
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