Bought item in a Toys-R-us. Was wrong color returned for exchange on used model for cheaper.
Instead of refunding me the difference or price they refunded me the full amount. She told me everything is taken care of and I can leave.
Am I obligated to return to store and pay difference they mistakenly refunded
You are not obligated to pay the difference. Most probably they didn’t make a mistake, and they are giving you the difference to keep their good name, and to make up for the inconvenience of you having to come back to the store, etc. Additionally, it would be similar to hashovas aveida that you are not obligated to do in this instance.