Specify what you think the amount of the credit should be, and explain how you calculated it.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We are returning <returned merchandise>. Instead of a refund, would you please credit the amount toward our <outstanding balance/future invoices>.
I have calculated the amount of the credit to be <$amount of the credit>, based on <credit calculation>. Please advise us as to whether this is accurate according to your records. Thank you.
encl: <List of enclosed items goes here>