Taking in mind that USDT virtually is USD, you will need to have an intermediate commodity to do that.
First, buy something highly traded (ETH, BTC, whatever) and transfer onto an exchange that offers cross-token operations with USDT (for example,
Cryptagio.com).
Trade the tokens against USDT, and then do the reverted operation to withdraw more fiat than invested.
Yes, this works if you simply buy cheap @ sell expensive any commodity, but trading tokens against a USD-tethered token multiplies your earnings by a few times, as token prices fluctuate over a fluctuating commodity (ETH in most cases).
