You can take the following steps if your customer owes you money for products or services provided by you but refuses to pay:
Clearly communicate the need for payment. Explain the terms of your transaction with reference to invoices, contracts or other documentation. Talk to your customer in a respectful manner about non-payment.
Write a letter of demand. In the event that talking does not work, you can send your customer a formal demand letter requesting payment. Write a letter in which you state the amount owed, the date by which payment is expected, and the consequences of non-payment.
It may be helpful to hold a mediation. For large amounts, consider using a mediator for a non-payment dispute. Both parties can reach an agreement with the help of a neutral third party.
Consider a debt collection agency for substantial invoice amounts. Be careful which agency you go with if you go down this route and try to find one that is registered to an official debt collection association. Keep in mind that this method will require you to pay the agency a percentage of the debt recovered.
Taking legal action may be your last resort in recovering the debt. The best course of action should be determined by your lawyer.