Sky Sports understands Southampton could miss out on around £11million if Gareth Bale is sold to Real Madrid.
When they sold Bale to Tottenham in 2007, they inserted a sell-on clause believed to be 15% of any future transfer fee.
However, when they found themselves in financial difficulty, Southampton waived that clause and agreed to receive around £1.5million plus Spurs youth keeper Tommy Forecast.
Forecast never made an appearance for Southampton and is currently without a club.
Saints can still expect a small percentage of a Bale sale to cover training costs - that figure is thought to be around two and a half per cent.
However, a potential sale that could have netted Southampton £13million will instead only yield around £2million.