Cancelling service with a security deposit

So i have decided to leave AT&T and I have a $300 security deposit that doesn't release until April of next year, correct me if I'm wrong but I'm assuming this is how it would break down:

$165-$170 for the early cancellation

The last bill I haven't paid and this month's

Then the $300 would be applied to those charges and then I'd pay the remaining negative balance correct?

Thanks, the sooner the response the better. I want out of this network ASAP!