SAINTS are set to pocket millions as Premier League giants Manchester United and Tottenham Hotspur battle to sign Mateus Fernandes.
The Portuguese midfielder, 21, joined West Ham from St Mary's last summer following the club's relegation, in a deal worth around £42million.
Fernandes was unable to keep the Hammers in the Premier League, with Nuno Espirito Santo's side brutally relegated in 18th place on 39 points.
It means the former Sporting CP star is set for a fourth club in as many seasons, with a price tag of up to £80m mooted to secure his signature.
As part of the deal to take Fernandes to West Ham, a future sell-on clause was inserted, guaranteeing Saints a percentage of any transfer profit.
The exact significance of the clause is unconfirmed, but it has been reported to be 15 per cent of the profit the Hammers make on the starlet's sale.
If he was purchased for £80m, that would mean West Ham made a £38m profit - seeing Saints pocket £5.7m, as 15 per cent of the £38m profit.
If it is actually a 20 per cent clause that Saints hold, they would take £7.6m in the event that Fernandes was sold to Man United or Spurs for £80m.
Even if Fernandes was only bought for £70m, Saints would be owed £4.2m for a 15 per cent clause and £5.6m if the clause they hold is 20 per cent.
Reports suggest that Fernandes is open to moving to either Old Trafford or Tottenham Hotspur Stadium - whichever club agrees a deal with West Ham.
Champions League giants Real Madrid, managed by Jose Mourinho, were also rumoured to be tracking him, but are said to have dropped their interest.