Upon the voluntary sale or conveyance of a unit, the grantee shall be jointly and severally liable with the grantor 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 grantee's right to recover from the grantor the amount of any such unpaid assessments which the grantee may pay, and until any such assessments are paid, they shall continue to be a charge against the unit which may be enforced in the manner set forth in § 2234 of this title. 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, the former unit owner's heirs, personal representatives, successors and assigns.