JazakAllah khair
Answer:
In the Name of Allah, the Most Gracious, the Most Merciful.
It will depend on the intention of the customer when he told you to keep the change. If he intended to give the change to you, it will be permissible for you to keep it. On the other hand, if he intended to give the change to 7-Eleven, it will not be permissible for you to keep it. If you are not sure whom the customer intended to give the change to, you should either seek clarification from him or you should give it to 7-Eleven to be on the safe side.
And Allah knows best.
Mufti Faizal Riza
9/1/2012