Can anyone help on this matter?
Easy - just send it back. If you have the private key of the USDT address, then it is a bitcoin address and it holds both bitcoin and USDT. just do a bitcoin transaction to send bitcoin and sign it with the USDT private key.
If you don't know how to do that, then just import the USDT private key into your favorite wallet. BAM~ you bitcoin will be there after it syncs.
Yes, so OP just send a support ticket to poloniex, polo will solve this question for OP. OP just be easy, don't worry.