Why sell p2p if you just had to convert it to the desired currency without any fees?

Good afternoon. Could someone answer a question for me?
I sold 8.3 USDT directly to another investor through FUNDS, I received the PIX and it appears as COMPLETED. But now I saw a complaint from the person I sold it to saying that they did not receive the USDT, asking me to release it. I am afraid of a scam, how can I check this?