Basically its your word against theirs, they believe the oil has gone in only because you have paid for 5.7 litres of it on the invoice. It is possible that damage of some degree has been done. If 5.7 litres of oil went in and now your 2 litres short you have 1 of 2 issues, its leaking out or its being internally consumed. Its also very possible that only 3.7L went in causing lack of lubrication and excessive heat which may have infact caused your now current oil consumption problem. I would pressure the dealer to prove to you they didn't damage your engine from a service mistake and get really involved in the diagnosis. Dealers hate it when they get to service their own product but dont give in to them! I take it the car now has a few trouble free k's on it before this. Its to easy for the dealer to cover their tracks now your car is using oil regularly, keep that in mind. Good luck!
Every man should have a V8 ute.....with a Chev badge.....and a Southern Cross sticker! Mods? What mods?