Explain Dispute of right as contrasted to dispute of interest?

I brought (2) phone with Verizon Wireless. And paid $ 99.95 for the phones. I cancelled the service after 5 days. Now, they are charging me for activation fees on this account in the amount of $ 104.00. They have been calling my home every 2 days requesting THE FULL AMOUNT. They phones were sent back and a credit was issued in the amount of $ 99.95. Now they are about to put this amount on my CREDET REPORT. Can someone give me advise ? My name is (William Joseph) I can be reached @ 706-992-2666 or @ jaydog357@hotmail.com