The "January window clause" is just saying he isn't leaving to go back to Napoli, doesn't mean he won't be sold. There is a salary issue, which Galatasaray requested whoever buys Osimhen, should pay for the entire season, so whatever Galatasaray paid so far to Osimhen, will be paid back to Galatasaray if this deal happens. But that's it, he has the right to leave if he is sold, he just can't be recalled to Napoli, those are different things.
But even with that in mind, United seems like they are not getting him, some news places denied the rumours and said United isn't interested so far, doesn't mean it won't happen but I think the deal is not really on the table right now. But if Galatasaray could get Kualo Muani, that means they are expecting Osimhen to leave, and they are looking for strikers at the moment, so it's clear that they are having doubts about keeping Osimhen until the end of the season.
Suddenly we have serious hype about Osimhen future in Galatasaray because current Manchester United situation is clearly indicating they will try their best for having this lethal weapon into their squad with things could be much better, and they will be able to have something good for the second half of this season but Galatasaray which is also having troubling time due to injuries can't afford to lost him but how things could be settled it is not clear while most chances if we have some pressure from Manchester United then we will be having this transfer done.
In case then surely we will be having Marcus Rashford could be also good for the Turkish league leaders or if they have Kualo Muani this is also going to be not bad while things could be under control for them in remaining season, and they will be able to have enough space for staying at the top and take good end of their season.