T Mobile Overcharges and will not fix their incorrect plan on my account to the correct one I changed. Being billed for incorrect plan Venice, California

On February 11, 2013, I changed my expired annual contract data plan for my Galaxy tablet to a pre-paid plan. The annual contract plan was $50 a month versus a $25 prepaid plan. 

We then received our bill since February 11th and our bill reflects the annual contract plan rate; in addition, to the prepaid plan rate. My fiance and I have tried numerous times to contact the store representative who did not update corporate records to reflect our changes. We have a receipt that shows the change to the pre-paid plan. After numerous attempts to Customer Care, they continue to defer responsibility to the store and have no record to change our monthly plan. The store will not take accountability for their mistake and indicated that they have no authority to update to corporate records? I would like to know since when do franchise systems do not communicate with corporate systems in a major wireless telecommunications company? 

Today, the manager, Frederick, of the store who has not helped in this situation avoided my calls twice. He told me he had no idea what is going on with my phone. We are now in April and are above $150 in backcharges that are incorrect. Since T-Mobile feels comfortable in not taking care of this situation, they feel comfortable in charging my account incorrectly and reporting me to collections.  Neither the store or Customer Care are willing to take accountability for their original salespersons mistake. 

I would these incorrect backcharges removed immediately.