Well you have 3 options.

1. keep it until next feeding day
2. try to return it to the store
3. kill and discard

Personally when feeding live I will put the rodent in with the snake (watching the entire time the mouse is in the snakes cage) if the snake doesn't show any interest after 10 minutes I will kill the rodent then place the dead rodent in the snakes cage overnight.

(If you are feeding the snake in a separate container I suggest that you stop doing this as it can stress the snake out)