Reverse Posted Purchase Invoice - Lot Tracked

Hello,

I am in a bit of a pickle, maybe you guys can help me out.

A Purchase order was created for an item (which is Lot tracked) for 6000 pieces. One of the staff members called the Vendor and agreed upon receiving 5800 pieces instead of 6000. However, the staff member forgot to change the quantity on the order. When the goods arrived at the warehouse, they received and invoiced all 6000 pieces. Goods were then moved to a different location and they were consumed. I cant do undo shipment, since it’s already been invoiced. I need to bring the quantity down by 200. I created a general purpose CM (for a G/L Account) to bring the amount of the invoice down for 200 pieces. How can I bring the inventory down by 200? I was thinking about doing an negative adjustment at the same cost of the original per unit cost. However, since the goods are lot tracked, I do not have a lot to apply this transaction against.
What are my options at this moment.