Our customer filed a claim on 01/27/2009 for an unauthroized EFT. Final credit was given on 02/05/2009. Merchant refunded customer's checking account on 02/10/2009. Bank revoked final credit on 02/20/2009 upon realizing merchant had refunded customer's checking account.

12 CFR 205.11 states that upon debiting a provisionally credit amount...blah, blah, blah.

If the bank had never given provisional credit but gave final credit and sent the customer a letter stating that final credit had been given, would the bank still need to sent a written explanation as to why the final credit was debited or does the written explanation only apply when provisional credit is given?

In this situation I send the customer letter stating that the merchant issued them a credit for a transaction that the bank had already reversed for them. Then state that you are reversing the merchant's credit, not the final credit from the initial dispute.
You are therefore not denying the Reg E claim (which you can't do once final credit is given).

We do the same as crowman3,