My lender tells me if I voluntarily return my car I will be liable to the IRS for 20% of the loan balance. Is this true?

The answer is that it depends on the circumstances.  What your lender is likely referring to is the assertion that, upon voluntary surrender of a vehicle, the fair market value of the vehicle may only be worth eighty percent (80%) of the outstanding loan balance, meaning that the Lender would be relinquishing (or taking a loss) to the tune of 20% of the loan balance.  This loss by the lender could be reported to the IRS as a taxable gain to you.  You should have your specific agreement reviewed by an attorney before entering into any such transaction.