Is it not the case that buying clubs accept buy back clauses because they get the player for less in the first instance and the buy back clauses pay them a guaranteed profit if enacted?
As example; a young player may be notionally valued at £20m but gets sold for £15m with a buy back clause set a £25m.
The buying club gets the player for less than market value, if the player improves and the selling club implements the buy back clause then the there’s a guaranteed profit.
It’s a win-win. The selling club effectively pays for an insurance that protects their interests if the players development suddenly accelerates. By accepting a less than market price, as per the above example -£5m, that is effectively a £5m investment, and then if buying back paying a further £25m (£25m minus the sale proceeds of £15m = £10m that would need to be covered by the players improved market value.
Having a buy back doesn’t necessarily preclude a bought back player from being sold on immediately, and I doubt that would ever be the case.
What more often happens, when a player improves and then is sold, is that the players original club gets a percentage of the sale price (% of sale is generally included alongside the buy back).