Is it considered a lie and does it negate the reward for charity if I give a vendor a large sum of money to buy something inexpensive, intending not to return for the change so that it becomes charity for her, while making her believe I will return?

This should not be done, as it involves lying. It also makes her liable for the remaining money, which has become a trust (wadi'ah) in her possession. If you do not return, she is religiously obligated to endeavor to deliver the money to you. If she cannot find you, she is religiously obligated to give it as charity on your behalf. This action burdens and inconveniences the woman. If you wish to give her charity, then transfer ownership of the money to her.