The reason why Man UTD have upped their bid is because they know Newcastle won't waste a second in securing his services so in order for them not to lose out completely, they brought money. So all it needed was for the Red Devils to get a competition for them to show they are serious in acquiring this player? Which means the money being asked wasn't an issue. I also hope they get Mbuemo because with unbalanced at the moment, they'd all the bolstering they can get.
Manchester United hasn't moved quickly in acquiring new players at the moment, compared to other Premier League teams like Liverpool and Arsenal. In fact, Manchester United were already behind last season, so they need to work hard to secure the players they're targeting and want for next season. Being slumped under a different manager the following season is incredibly painful for Manchester United and its fans. So, raising their offer for one of these players also demonstrates Manchester United's seriousness in acquiring a great player like Mbuemo.