The credit is only when they pay invoices within terms. They have been late on every invoice so there is no credit now. They have to pay the full contract value since they have been late payers. Please communicate that. Once they start paying invoices on time then for 1Mn paid on time a 100,000 THB credit will be raised. This is only for invoices on the original contract and doesn’t apply to change orders. It’s their fault that they did not pay on time despite repeated reminders.