After Martial scored his 25th goal for the club, United will have to pay the Ligue 1 side an extra £8.5million.
The 21-year-old initially arrived at Old Trafford for £36million in 2015, making him one of the most expensive teenagers in English football.
But after the Frenchman slotted home his 25th goal for the club, United will have to pay the Ligue 1 side an extra £8.5million.
The overall transfer fee for Martial could potentially rise to £63million as there are more clauses still active in the transfer.
The Red Devils will have to hand over another £8.5million if he picks up 25 caps for France – which would take the overall fee to £53million.
And one final clause is that they pay will Monaco an extra £10million if he is shortlisted for the Ballon d’Or.
Martial has struggled at times this season under Jose Mourinho but has still managed to net his 25th goal for the club.
But next season if Martial rapidly improves then it could cost United just short of £20million.
He is only ten caps away from 25 international appearances and if his form improves he could get shortlisted for the Ballon d’Or.