An SPA names the products, the customer or job, the authorised price and a validity window. It exists because the supplier wants to win a specific piece of business without moving their published price for everyone.
The leak is almost always the link. Where the SPA is applied as a manual price override at order entry, the connection between the sold line and the authorisation that justified it is broken at the moment of sale, and raising the bill-back later becomes archaeology. Distributors typically trace under 70% of SPA-priced lines back to their authorisation.
Treating the SPA as an object the order references — rather than a price the order copies — is what makes the recovery claim automatic.