Aston Villa signed the striker, 25, from the Bees last summer in a heavily-incentivised deal.
Brentford received £28m up front for Watkins, with a further £5m in add-ons.
However, Exeter also stand to pocket a huge chunk of cash after the striker's inclusion.
Watkins worked his way through the youth ranks at the Grecians before joining Brentford in 2017.
That means, every time Watkins is capped by England - up until the undisclosed limit - Brentford will give Exeter £150,000 of their £1m share, with the Bees keeping £850,000 for themselves.
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