Anthony Martial's goal against Newcastle cost Manchester United £8.7 million.
A clause in Martial's transfer agreement from Monaco in 2015 guaranteed the French club an additional payment of €10m if the forward reached 25 league goals before 2019.
Martial reached the mark during the 3-2 win over Newcastle, scoring the second as Jose Mourinho's side battled back from two goals down.
Monaco will also be paid add-ons of €10m if Martial reaches 25 caps for France -- he's currently on 18 -- and if he is nominated for the Ballon d'Or.
If all the clauses are met, Martial could end up costing United more than £61m.
The 22-year-old has scored 25 goals in 91 Premier League games in a little over three seasons at Old Trafford.
He has scored 38 goals in 144 games in all competitions, including a goal against Liverpool on his debut in 2015.