My institution is looking for a letter to send to customers when we receive a merchant rebuttal on their debit card dispute so the customer can respond and win the dispute, along with the documentation that the merchant provides. We also need a letter to send to customers when we are just requesting more information. Can anyone provide sample letters for these situations?

These would seem to be easy templates to create based on your need, rather than finding one another bank has and then having to customize it.

Reg E imposes requirements for a written notice when you deny a claim, but otherwise "written" is typically not a requirement. The phone or email/online banking being more of a secured method, may be faster and more timely.

Since this is not required and must be customized, I'd recommend creating it rather than re-creating it. It'll be faster and exactly what you want.

Identify the customer
Describe the issue
Describe what options the customer has
Define a date by which time it must be done
Who should they call with question

I should add, this method will also allow the bank to tailor the use for phone calls as well, when desired.

If you are uncomfortable drafting these letters/notices or related telephone scripts, get your beginning thoughts down on paper. Then enlist someone in your organization with effective writing skills (you might find them in your marketing department) and ask them to offer suggestions. Be careful to make sure that the essentials of your message are not compromised in the process.

You don't need to make these communications fine prose. They should be short and businesslike, and convey what you need to tell the customer. Nothing more.

One other thought. These are coming from the bank, not from you personally, even if you sign them. Avoid adding personal touches or inspirational messages in your signature block.