Doubt as to liability means that a legitimate doubt exists that the taxpayer owes part or all of the assessed tax liability. A decision by the Internal Revenue Service to accept or reject an Offer in Compromise - Doubt as to Liability rests primarily on a reconsideration of whether or not the person assessed was responsible for and willfully failed to pay the tax in question. The taxpayer must offer a dollar amount greater than zero. The amount offered may be a cash or deferred offer, but must be paid in full within 90 days if accepted by the IRS.