Specify the check number, amount and what it covers. Whether an oversight or poor financial management, an apology will help preserve the relationship.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Enclosed is check number <check number> for <$amount of check>. We apologize for <problem> invoice number <invoice number>. This check should cover the full amount still owed to you on this invoice.
We're sorry if this has caused any inconvenience and thank you for your patience.
encl: <List of enclosed items goes here>