The fact that Slot is being called head coach and not manager makes me wonder why. It may be to limit his role to coach compared to Klopp who managed and coached e.g he had a say on player transfers - both ways btw.
I'm certain Klopp was head coach when he first arrived but changed to manager as part of a reshuffle
I don't mind the manager / coach having a back seat when it comes to transfers, it makes for a better long term strategy should the manager / coach not work out (hence is insisting on the new manager playing 433). However, they should always have a say and no player should be forced upon them.
If all parties agree to the sign, sign him. If anyone disagrees, don't sign them. Simple.