I wish to take this opportunity to thank you for allowing us to represent you in the [describe] matter. In order to complete this matter, I have [briefly describe what you did]. Further, to facilitate closure/termination of this matter, I will [outline any final matters you will take care of]. In addition, you will need to [outline anything that the client is responsible for at this time].
Since this matter is now closed, I suggest that you keep all information relating to the matter in a safe place where you can easily locate it. I am closing my file and I am returning [or forwarding to successor] your original [records, documents] related to your case. My firm will store your file for [number of years] from now. The file will then be destroyed unless you [or successor] request that we store the file for a longer period of time. If you wish me to store the file for a longer period of time, you must instruct me to do so in writing within [insert #] days of the date of this letter.
We are pleased that this matter has been concluded to your satisfaction. Thank you for allowing us to represent you in this matter. If we can be of further assistance on this or any matter, please let us know.
If sending a version of this letter to a beneficiary, consider noting something like the following:
Please be advised that my work in connection with the trust/estate administration is now complete. [Or was completed on X date]. (Do not leave it more than a few days to send this letter, however).
Please note that I am holding a reserve of [X amount] in the event that [certain expenditures may be needed]. However, as noted my substantive work is complete. Please let me know if you have any questions or comments at all.