Here is a sample letter you can use to request a change in your payment date:
sample request letter for change of payment date
I am writing to request a change in the payment date for my [invoice/account/contract, etc.]. Currently, the payment is due on [current payment date], but I am requesting to change it to [requested payment date].
The reason for this request is [reason for request]. I understand that this may be an inconvenience, but I hope you can understand my situation and grant this request.
I will make sure to have the payment processed on the new date, and I appreciate your consideration in this matter.