If your customer accepts that money is owing and is willing to make arrangements for the debt to be repaid consider creating a debt management plan. Record an offer to pay off the debt by regular fixed instalments in a Debt repayment plan letter. If you're the recipient of such a letter and decide to accept the proposal, you can consider doing so using a Letter accepting payments in instalments.
If you want to formalise your agreement to pay in instalments in a form of a loan agreement rather than a letter, you can use a Promissory note, which sets out the names of the parties, the amount due, instalment amounts, provision for interest and the effect of late payment.