It's probably tough to argue. Not sure what the formula is for dividing up revenue for non-partner flights, seems fairly reasonable it's whatever chunk of revenue goes to the other carrier. That info. Isn't necessarily easy to find, and certainly isn't on the e-ticket receipt.
The best option in the past was to look at the fare breakdown on the Saudia site. I don't think there is any confirmation if this info. can be found there anymore since the interline agreement ended, and even if it is, not sure how long that remains there after all segments are flown. Furthermore, not sure if the breakdown goes to specific segments unless there is a fare break. I guess the only way to find out is to try - OP can attempt to go to Saidia.com and try to bring up the ticket there (using ticket number (PNR definitely doesn't work) and last name). If OP doesn't understand the lines there, near the bottom below the actual itinerary, then someone can probably decipher if it gets posted here. Again, only if it's still available, but certainly worth a shot.