Overcharge on bill

I bought an iPhone 8 for my brother (who is partly disabled) on my family plan. He was unable to use it and I returned it to an AT&T store in downtown San Diego within four days. The AT&T store assured me my bill for use of the new phone (895 area code) would be prorated. However, AT&T is now billing me for an entire month for the use of the 895 phone. Please help me reduce my bill for February. Thank you, John (edited per community guidelines).