Trade-in valuation not fully paid by TMobile

I was given a valuation for my Z Fold3 for $500 and I accepted. So I traded for the Z Fold4, sent in my old one, and that's it - it should all be done and setup. They only gave me $385 credit on my bill and didn't give me the full $500. When I was texting with their support through their chat, she led me to believe that the rest of it would be credited on the next billing cycle.

Can anyone confirm that this is true?