Your agreement with the agent should specify the payment arrangements, including when payments will become due. If the agreement does not, then it will be assumed that normal practice applies.
Typically, the agent's commission becomes due either when you have received payment or when you supply the goods (or services) to the customer.
If the agent is self-employed, and your agreement says nothing to the contrary, then the commission does not have to be paid if the sales agreement with the customer will not be executed through no fault of your own: for example, where a customer refuses to pay for the goods (or services) for no good reason.
