I remember reading many years ago that this receiving a percentage of any transfer fee for a player is a very common clause & is inserted into most lower division managers contracts.
Its supposed to make up for the lower wages they receive & is an incentive to try & get them to bring through cheap/youth players & rewarding them for when they’re successful.
I’m pretty certain there was a bit of uproar many moons ago when Latics accounts showed Bryan Hamilton getting a massive wage one year & it was coz he had this clause in his contract
I know Blackpool are no longer lower league but they aint big payers & I’d imagine their contracts are loaded with incentives like this