The fee for Anthony Martial could rise to £58m (€78.7m) if he goes on to become one of the world's best players.
The French striker secured his deadline day move from Monaco, on Tuesday, and United will pay £22m up front.
The Premier League club will then pay £3.5m over the next four years - adding up to £36m (€48.8m).
However,
according to French broadcaster RMC, an additional €30m of add-ons could make the 19-year-old United's second most expensive signing of all time [Angel Di Maria cost £59.7m].
Monaco could bank another €5m each time Martial finishes the Premier League as top scorer - to a maximum of €15m.
There is another potential €10m if Martial wins the Balon D'or while he is with United.
More realistically - for now - Martial's former club will get €5m extra if the striker wins the Golden Boy. That award is given to the best player in Europe under 21 and decided by a host of top European football journalists.
Previous winners of the award while at United - Wayne Rooney in 2003 and Anderson in 2008.
Asked about Martial on Sky Sports, Thierry Henry said, 'He has pace but can he score a lot of goals? I actually don't know because he has only played 52 [senior] games.
'There are a lot of boxes you still have to tick with him so it is a massive gamble but you have to do that these days, especially in the situation United are in.'
As for Martial, he claims that the pressure will not get to him.
Here is his first interview after penning his massive deal with United.
