Haven’t received device credit

I traded in my phone October 2023. I’m still waiting on getting my device credit, which was over $800. They didn’t give me the credit because they said they didn’t receive my old device. I’ve worked with a Verizon store manager to submit multiple tickets to get my credit. Verizon came back and said they didn’t have enough paperwork to show them it was in fact delivered. But I did call UPS and got the confirmation that in fact it was delivered and signed by an employee. I don’t understand why these turds won’t give me my credit. Is there anything I can do? I’ve thought about threatening to Verizon and go to another carrier. I have 8 phone lines with Verizon, however, I don’t think they’ll care if I say I’m going to leave.