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.
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.
However,for the price settlement has to be agreed by RMD and TOL.
If you have any question or I am wrong please let me know krab.
Thank you.