Daley Blind made 143 appearances for the Ajax side before joining United for £13.8million in 2014.
The 28-year-old was linked to a move to Roma in January, but he has now shown willingness to join Ajax.
Blind has a year left on his contract, after the club extended his deal by 12-months until the end of next season.
According to the Telegraph, United have asked for a fee for Blind, which Ajax would be unable to fulfill alongside the player's wages.
Manchester United have since waived the fee in order to allow the Blind move, allowing Mourinho to bring in reinforcements for the coming season.
Blind would be taking a wage cut if he were to return to Ajax.