After check solution proof from the document B/L report to Thailand Customs Department is 391 CTN.
The importance issue that we are report invoices matching with B/L and export customs entry to Thailand Revenue Department is 391ctn.
Then for the smooth process they should be matching QTY(invoice +B/L + export entry). No need to pay penalty fee incase Revenue Dept. check.
You are correct incoterm of delivery is Ex works, but the goods belong to GG HK only 391 ctn not 396 ctn.
Because the finally all of processing proof the goods is 391 ctn.
The best way process deSter PRB issue Credit note to GG HK and GG HK issue Credit note to Customer = 5 ctn as loss QTY (as Etienne inform the Credit not issued to Customer already).
And Yougstar pay 5 ctn to deSter PRB for offset which the goods 5 ctn as loss.
The finally you will be get Credit note from us.
Please let me know for problem with your Revenue Department so we will try the best solution.