[Your letterhead, if desired; if not, your return address]
[Date of letter—month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I am writing to express my sincere apology for not paying the money that I owe you at the time when I had promised to pay you. As you know, I am the type of person who always follows through on what I promise, including promises about repaying debt. Unfortunately, some money that was supposed to arrive to me did not, and this is what has caused me to be unable to pay you.
I plan to pay you the full amount that I owe you within the next 30 days, or sooner if I have the money before then. I want to pay you back as quickly as I can and do not want to create any strain in our relationship because of this late payment. Our friendship is so important to me, and I thank you for understanding and for accepting my apology.
[Sender's first and last names]