Sanchez only signed for the Dutch side last summer, penning a five-year contract after a 5million euro fee was agreed with Atletico Nacional, but he has already become an integral part of the first team, making 32 Eredivisie appearances and scoring six goals.
The Colombia international received rave reviews for his performance in the Europa League final defeat to Manchester United and there are said to be a number of clubs keen on his signature.
Tottenham are believed to have pushed ahead in their efforts to sign the 21-year-old centre-back although the £35million valuation is a stumbling block, with Spurs only willing to part with £25m.
Negotiations are reportedly on-going between both clubs but Mauricio Pochettino will have to move quickly to land the defender with several other top European clubs readying offers.
Barcelona, Chelsea and Crystal Palace, who are managed by Dutchman Frank de Boer, are all keeping an eye on the situation and could make a bid in the closing stages of the transfer window.