My first question would be 'how did the other account get RECONCILED' if it shouldn't have been in that account? Seems like the other account needed it.
What kind of transaction was it? How you fix it, will depend on what kind of transaction it is, and where it originated from. Please provide more details. (Typically, if a Payables, check, you can void it in Payables, and then rekey it as a manual check in Payables in the right checkbook ID. Then back rec will put the check in the correct checkbook, and a negative check in the original checkbook, that you must research and key an offset to it, etc if it's not needed, but appears it was if they were able to reconcile and didn't key an adjustment in that recon to offset it.)
However, it seems to me that the checkbook it went to, needed it afterall, or they wouldn't have been able to reconcile. You will want to first check that reconciliation to see if they keyed an adjustment or what they did in there to be able to reconcile.
I hope the above information is enough to get you started. Let me know if any more questions.
Cheryl
Microsoft Dynamics GP Support