Google Play has a peculiar habit of making a pending charge nearing the end of your trial. However, as long as you cancel before the end of the trial, the pre-authorized charge will be removed.
If there is an issue with this, we recommend contacting Google Play directly and asking them about this problem. To contact Google Play, simply go to this link and press Get Help towards the bottom for Google to call you.
Just a note that Google refers to this as an authorization that needs to be cancelled rather than as a refund.
Need further help? Please email us by going to "Send Us Feedback" in the settings menu of the app or writing to firstname.lastname@example.org.