The Sun says he left United with a severance package worth around £4million.
The Dutchman, 28, rejoined Ajax last week for £14.1m — with a potential £4.4m more in add-ons to come.
United and Ajax came together to pay up the final year of Blind's contract, which was factored into his transfer fee.
Jose Mourinho has been promised around £10m from the sale towards his own summer kitty.
The Portuguese wants to bring in two more players having signed only midfielder Fred and young right-back Diogo Dalot.