Upon the voluntary sale or conveyance of a unit, the purchaser shall be jointly and severally liable with the seller for all unpaid assessments for common expenses which are a charge against the unit as of the date of the sale or conveyance, but such joint and several liability shall be without prejudice to the purchaser's right to recover from the seller the amount of any such unpaid assessments which the purchaser may pay, and until any such assessments are paid they shall continue to be charged against the unit which may be enforced in the manner set forth in section three of this article: Provided, however, That any person who shall have entered into a written agreement to purchase a unit shall be entitled to obtain a written statement from the treasurer setting forth the amount of unpaid assessments charged against the unit and its owners, and if such statement does not reveal the full amount of the unpaid assessments as of the date it is rendered, neither the purchaser nor the unit shall be liable for the payment of an amount in excess of the unpaid assessments shown thereon. Any such excess which cannot be promptly collected from the former unit owner may be reassessed by the council as a common expense to be collected from all of the unit owners, including the purchaser, his successors and assigns.