To be honest, playing for a big team like Manchester United is a special experience, so I think Osimhen would still be interested in moving to Old Trafford. Especially if Manchester United makes Osimhen the highest paid player at the club, that would be hard to refuse. Rather than staying at Galatasaray or returning to his parent club Napoli, it would be better to move to Manchester United. If Arsenal, Chelsea, Liverpool, or PSG join the competition to get Osimhen, Manchester United will likely fail to secure him because those clubs can offer Osimhen to play in the Champions League next season. Let's see first if Manchester United can win the Europa League trophy, which might make Osimhen interested in joining Ruben Amorim squad.
I agree that playing for Manchester United is special chance that many players including Osimhen would love to have. If Manchester United offers him a lot of money and makes him highest paid player that would be hard for him to say no to. But other big clubs like Arsenal, Chelsea, Liverpool and PSG might also want Osimhen and they can offer him chance to play in Champions League which is big deal for top players. If Manchester United wins Europa League that might make Osimhen more interested in joining team. It will be interesting to see what happens next.