Upon the sale or conveyance of an apartment, all unpaid assessments against a co-owner for his pro rata share in the expenses to which section 18 refers shall first be paid out of the sales price or by the acquirer in preference over any other assessments or charges of whatever nature except the following:
(a) Assessments, liens, and charges for taxes past due and unpaid on the apartment; and
(b) Payments due under mortgage instruments of encumbrance duly recorded.