Mary Francis,
What a very timely question. With rates dropping many may face this issue.
The charge can be passed to the borrower. The borrow requests the change. You agree, subject to the condition that they pay the fee. Everything is detailed in the note or modification agreement. If you use a new note, the transaction is a refinance and disclosures are required. If the existing note is modified, no disclosures are required.
If the transaction is a refinance subject to TRID, the fee could be disclosed on the Loan Estimate and on the Closing Disclosure in one of two ways. First, if the fee is retained by your bank it would be disclosed as discount points in section A, since the fee is paid to the creditor to reduce the rate that otherwise would have been imposed. The second option is to disclose the fee as a Service You Can Not Shop For in Section B, since the fee is actually paid to a third party.
Your question indicates that the fee is ultimately paid to the investor, which makes the second option most responsive to your question.