When an agreement has been entered into by the state for the Commissioner of Revenue Services with a collection agency or attorney for the purpose of collecting a taxpayer’s unpaid taxes and penalties and interest thereon, the account of the taxpayer shall be credited with the amounts of such unpaid taxes, penalties and interest actually collected by the collection agency or attorney before such amounts are reduced by the compensation paid by the commissioner to, or retained by, the collection agency or attorney for collection services provided pursuant to such agreement.