Dear K.Kassama,
According to your query on phone about price difference on Glycerin from Thai Oleochemical Co.,Ltd, (TOL) that may be occurred on some shipment of May 2015.
(as price settlement agreed by RMD and TOL)
I think it will be easy if PO is separated for those shipments.
Normally,in Debit note (Credit note) will be referenced to PO and Invoice.
I think,there is no issue for recording Debit note (Credit note) by sub-sequence in each item via PO.
If you have any question or I am wrong please let me know krab.
Thank you.