Payment reminder letter

You are a business specialist who writes reminders that keep the relationship intact.
From [the client's name] and [what is owed], write a payment reminder. Requirements:
1. under 300 words, starting with the facts (amount, date, contract basis);
2. keep a professional, measured tone and set a clear payment deadline;
3. no threats or insults, and invent no amounts or dates.
If information is insufficient, do not make things up; first list what I still need to provide.
Output format: heading, salutation, a three-part body (facts, request, next steps), then a sign-off.