The Denmark international striker has also agreed personal terms with Milan.
Milan will pay €24m plus bonuses to Sporting for Harder, who will also be paid €2m-a-year for the next five seasons.
TMW says the move is now waiting on Sporting to sign a replacement for Harder.
Sporting are in negotiations with Panathinaikos for Fotis Ioannidis with a difference of €2m currently standing between an agreement.