Stripe API Balance Deduction

I am trying to implement call features in my project.After successful call i need to deduct the amount according to number of minutes spoken.Lets say the user has an appointment for 50 min and total amount is 500 usd. but the main problem is how can i verify if the caller has enough amount that i.e equal to 500 or more before starting the call so amount will be received according to the minutes.I have been searching answer but couldnt find it.I have used STRIPE for that purpose

via Chebli Mohamed