What can i do about this with T-mobile?

Okay guys i would appreciate if people are polite about this question please here's the thing on October i went to T-mobile to buy an iphone and well i was about to pay for the phone and plan, but then i found out the guy was giving me the wrong phone… So i told him to give me my money back and cancel everything else… So he "did" and then on november i got the iphone i wanted and went and connected it with them on a prepaid plan and everything was fine till i goto a letter that i owe t-mobile 100 something money for a phone that i didn't even end up getting and a plan that i didn't even use for 3 months
Added (1). lol destroyer of hearts HELLL NAW i need to pay my next semester in college by the end of the month xD

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