Usually what I do when they offer gum is I smile and say I don't chew gum and patiently wait for my change. Usually then the offer chocolate, and I say no thank you I would prefer the money. Actually the other day my bill came to 9.75QR and I gave the guy a 10 note. He took it and began to move on to the next customer. I then asked as nicely as I could, "can I have my change?" He stammered a bit since it had not even occurred to him to offer change. Eventually he gave me a 1QR note. :)
It's the principal of the matter, not about the actually cash.