How to do reversals in SLA?

I have setup custom entries in SLA to create accruals for earned discounts on the invoice event and reversals, if the discount is not taken, on the receipt event.

Now i need to extend this.  If an invoice is applied to a receipt and Create Accounting is run my custom entries are created.  If at a later date, this invoice is unapplied

from the receipt, SLA will reverse the earned discount that was originally created.  I do not see a JLE or JLD for this reversal. How is it being done?

Bottom line, i need to reverse my entries on the receipt event also. Is there a source that can be used to identify the invoice that was unapplied?