The invoices always stated "cash discount of 2% if...

The invoices always stated "cash discount of 2% if payment before <date>". We have therefore always paid the invoices immediately on the assumption that we would receive the discount. When we asked about this afterwards, they were simply saying that we should have deducted the discount from the invoice itself, and that reimbursement was no longer possible. Who does this now ??! Promise people a discount and they will blow it through the nose afterwards. I will never go with them again!