Is disagreeing appropriately manipulative?

I was at the mall with a friend and our children. My child asked for something and I said no. So he disagreed appropriately and I ended up changing my mind and giving him what he had asked for. My friend said I had allowed him to manipulate me because in the end he got what he wanted. So this got me thinking if it’s true. How could I know if my child is using disagreeing appropriately to manipulate me or the situation?