When the Carry forward VSB Period is run one of the things it does is to archive sold vehicles once they meet the archiving criteria.
To be archived vehicles must,
Have all Specification lines invoiced and all costs received.
Have been at X for at least the number of days specified in the VSB System Parameters
Any Trade-ins on the vehicle to be archived must also be ready to archive.
The overallowance account gets one side of the posting from the invoiced vehicle and one side from the trade in. If one of the vehicles is archived before the other a balance will be shown in the VSB column on the Reconciliation Report.
A balance in the VSB side of this Reconciliation Report in the Overallowance Clearing account is not really an issue, the important thing is that the NL side is zero.