A customer notified us that they were unable to locate a merchant refund they were anticipating to their account. Our Error Res investigation revealed that for some reason, the EFT had been received in Oct but had been omitted from their Oct statement. Our records also show the customer then transferred the funds from the account the same day. We've confirmed an error occurred, but the customer already received credit for the funds. I'm told we don't have a way of issuing a corrected periodic statement. Do we need to do anything else?
And yes, we are fixing this issue so it does not happen again.

"we don't have a way of issuing a corrected periodic statement"

Ever heard of something called Microsoft Word?

Sorry. I am sure that sounds like I am being a smart a$$, but there is always a way. If the statement you sent does not balance, I suggest providing them a manually prepared statement.

We could probably do that, but we cannot upload it to the platform in which we electronically deliver the statements. Still, better then nothing, I suppose.

when in doubt, mail it out. with a letter of apology and also thanks them for bringing this to your attention and that you are putting steps in place to make sure this can't happen in the future. that likely goes a long way in appeasing a client.