Customer rebates are a liability that builds continuously and settles periodically. Accrued from an estimated rate on aggregate volume, they produce a true-up at settlement that nobody forecast; accrued from transaction detail against the actual agreement rules, the variance collapses below a few percent.
They are also a pricing instrument in disguise: a customer on a 4% annual rebate is a customer whose real price is 4% lower, and if your margin reporting does not show that at the line, your commercial decisions are made on the wrong number.