They don't expect anything, they have a problem, United has some sort of a problem, they think it could be beneficial for them to have Mkhi and that those 6 months could improve his market value for United to sell this summer so they make an offer, not an ultimatum.
Kinda the same with Perisic, you might think you were offering more than what he's worth, I think it was almost impossible for them to get an equal player to Perisic with the money from the United offer, he's been great this season for them, probably better than any 40 or 50M player they realistically could sign.
If there's no good offers for Mkhi then its either give him more chances, bench him the rest of the season or give him an opportunity to improve his value at Inter. Of those 3 options the Inter one is probably the best, when managers bash on a player for almost 2 years you can expect teams to not be too hot for them when you want to sell them.