Add/post the missing credit note.

Oh, wait, now I see.  The credit note exists.  Just do a write-off batch.

I've never had to do a write-off batch before. Is it possible to even do it to a credit note?

You can write off anything.

But there is nothing to do it to. The credit note remains as a non day-end posted transaction. 

If it never made it to A/R, then add it to A/R manually.

Tried that before. Doesn't work since that CN still remains floating. 

And it will remain unless you do data surgery to remove it from OE.  Do a manual credit note in AR.

And it will remain unless you do data surgery to remove it from OE.  Do a manual credit note in AR.