Billing and customer service

Back in November I traded in 2 iPhone 16 max pros for 17 max pros. I have yet to be credited for one of them which I received a letter in the mail that assurant received them both but returned one do to its condition which I understand. The credit has not been received and I was assured all through December that I would receive the credit and also these last couple of days. My bill is now more than $1,200 because of not receiving the credit and I was given an extension until January 17th to pay that amount which is too high for me. I spoke to an agent on the American side today and they told me that customer service should have worked a plan for me to pay it off in months until my credit is received . I need this taken care of.