What needs to be in an acknowledgement of debt letter?

Very rough language, but the concept is what I have used in the past:

We (ABC Company) are authorizing a one time over-ride of our payment terms on products and services rendered to You (XYZ Company). Based on discussions between us, we have agreed to adjust the following outstanding invoices as follows (insert section showing current to new totals). These revisions will remain in effect as long as payment is received by December XX, 2013. If payment in full is not received on time, all invoices (whether partial paid or not) will be increased by 15% per month for delay in payment from this agreement. This agreement does not change payment terms for future invoices and You are advised to review the products and services received from us and notify us in writing within XX days of receipt, of any disputes to avoid future late fees or disruption of service. We appreciate your business, but as a fellow business owner You understand the need for us to be paid for our services in a timely manner. Thank you for allowing us to work together to resolve these outstanding issues. The signature and date of acceptance by an owner or officer is required at the bottom of this letter to confirm your receipt and agreement to these terms.

Make sure your invoices and/or your SLA is clear with payment terms and dispute resolution. Then put someone on the case to call when a future item is received by them to make sure they have no issues, followed up with a nice email thanking them for taking the time to confirm that there were no issues with the order. Depending on what you are providing them, you might see about mandating that future invoices are paid at the time of order via credit card, and limit their credit balance to avoid having them get out of control in the future. If they threaten to leave you and do business elsewhere, don't be too concerned. Your current grief becomes the next supplier's grief if their payment terms aren't very controlled.