bill overpaid I use the negative balance on my phone bill to buy a phone

As far as I am aware, no.  You can buy a device on EIP, pay the taxes and fees up front, and then ask Customer Care to transfer any extra you have overpaid your account over towards the device balance of the device but you can't just go.imto a store and buy a device with extra funds you have overpaid to.your account.

I suggest contacting Tmobile to confirm the negative balance and that Tmobile isn't in the process of refunding it.