We had a similar experience in July. We were walking from our
hotel down near the stella show and going to the canal. The grape man approached my mum, son and his friend. I was walking on a little, he gave the one or two grapes and my mum thanked him, at that he gave the three of them a little tray with a half dozen grapes that were only fit for the bin. Again my mum thanked him (to be polite) and we started walking off, he started shouting at us in Turkish and very obviously swearing at us (we learned a few sweary words). My dad walked back and asked him what the problem was.
He started shouting "lira, lira" my dad asked how much it was and he said 5 lira, we thought that was in total - but that was each! When my dad opened his wallet the guy had his hand straight in and was grabbing my dad's money. I thought my dad was going to flatten him!
Needless to say, he only got his 15 lira and a mouthful of Turkish abuse - which I think shocked him
So be aware, if anyone was to offer us anything in future, we will just say a polite no thank you and move on.