I guess Milan wanted it to be an optional. As long as Hojlund meet several condition, they can permanent him or return him. However, seeing the case that happened between MU and Chelsea, i expect similar thing to that. I meant Milan take him as a loan from MU, and once he doesn't meet their expectation. They will return him back to the MU.
Sending him as a loan to the Milan is a big lose for MU, though his salary will be fully covered by Milan. They want him leave as a permanent transfer.
Manutd could add obligations to buy in his loan clause to Milan if truly they want to get rip of him but that won't be a brainer. What Chelsea and Manutd had with Sancho was something different as there was no obligation to buy in Sancho loan clause, it was optional. If Milan are getting him on a free loan, they would cover his salary and bonuses.
Nobody knows if Milan will be a perfect club for him to improve himself and bring out the best in him. Things are not working for him in Old Trafford.
He's needs a place with less pressure to unblock his full potential and AC could be that place.