When municipalities choose tax collectors, they may agree what sum shall be allowed for performance of their duties. If the basis of compensation agreed upon is a percentage of tax collections, such percentage shall be computed only upon the cash collections of taxes committed to him. Tax liens filed but not discharged prior to the time that the tax collector is to perfect his collections and the amounts paid by the municipality to the tax collector upon the sale of tax deeds shall not be included in computing such percentage. Nothing in this section shall be construed as relieving the tax collector from the duty of perfecting liens for the benefit of the municipality by one of the methods prescribed by law in all cases where taxes on real estate remain unpaid.