A demand letter is a letter given to someone that owes you money.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
This letter is our formal notice that you are default on your contract to repay [Exact Amount Due], which was issued [Date of Issue]. If this amount is not paid within the ten business days of this letter's date, we will be forced to seek legal counsel and exercising our rights to enforce collection of amount duet. This could include not only legal proceedings against you but also any interest and legal expenses.
We want only to receive the money that is owed to us. This amount has been overdue since [Date]. Please contact us as soon as possible to discuss how repayment will occur or if you have any questions or concerns about this owed amount.