We understand the concerns of most Galatasaray fans but we also have to realise that Osimhen is only a loan player who can leave at any time.
Manchester United might be serious about Osimhen and they've made an approach much more likely to sign him and it really depends on Napoli's decision. Galatasaray may need to buy another striker if Osimhen really leaves because Batshuayi is not one of the best strikers ever, especially with Icardi's injury, so with conditions like this, the club will definitely consider buying another striker as a step to maximize the front line to be more solid.
"Leave anytime" isn't how contracts work, if Galatasaray doesn't want him to leave, they do have the right to keep him, there is no January exit clause at all. Its been established and known ever since he joined, so if Galatasaray wants to keep him, then they can. But at the same time, who would want to keep a player who wants to leave? That would be stupid, so its all about Osimhen, but so far he seems like he isn't leaving, well not for United at least, he may leave, but probably for another team, like maybe PSG, which looks like a bigger candidate.