Warranty void for invalid serial number on receipt

My JON# ***, I called HP International to check the warranty status of my clients HP laptop, HP requested the serial number, and when trying to discover the serial number i had found it under a black sticker which had scratched a bit of the serial number off. I then got my serial number by giving the model number with half the serial number to HP and they relayed a serial number back to me, and then advised me to go to Ensure Services for the laptop to be repaired under warranty as the laptop was purchased in Dec 2014. When i booked the laptop in at Ensure i then received a call from a guy that said the windows installed on the laptop is not the original windows it got sold with, they then assured me that they can fix it for R300 + R42VAT. When i told them that the laptop is still brand new they requested the PROOF OF PAYMENT (Receipt) from the dealer my client purchased the laptop from which is in Dubai, I sent them the INVOICE twice after they requested it with the serial number that is recorded on the Invoice, now ensure says that the serial number is invalid. HOW is the INVOICE with the RECORDED SERIAL NUMBER INVALID? How is this matter going to be resolved.