You are wrong there. This was the deal and ive lifted this straight from Sky sports, although its the same in other press outlets as well
'The document says the initial fee of 50m Euros (£38.5m) for the player is to be paid in two instalments - €20m (£15.4m) within three days of his transfer, and €30m (£23.1m) on July 1.
There are then three further potential payments - €10m (£7.7m) if the player scores 25 first-team goals for Manchester United, €10m (£7.7m) if Martial makes 25 appearances for the French national team and €10m (£7.7m) more if the player is ever nominated for the Ballon d'Or award.
There is also a sell-on clause inserted in the deal, with Monaco due to be paid 50 per cent of any transfer fee between €60m and €100m United receive for the player'