Negociating skills of the two year old.

This is Haylee she is two. She has recently mastered potty training. She doesn't need help but she still wouldn't go by herself. She would track me down to tell me she had to go potty, I would say "okay go" then she would say "come wit me". Last week I went "wit" her and started to help her up on the potty and she told me she could do it herself. so I asked her why was I here? she said"because I love you" how do you argue with that?
This week I decided it was time for her to start going by herself. I told her every time she went potty alone I would give her two m&ms. She was excited and told me when she had to go and then went by herself. It was a great idea and it was working as I had planned.
The problem came after ward, when she came for the m&ms. I was careful not to let her see where I had gotten them and gave her two m&ms, but then she negotiated four?
Don't ask me how the negotiating skills of the two year old trump the physiology skills of their mom but it is written somewhere,it happened to other mothers too.