Section 21. If a license is granted by a probate court for a sale or mortgage of real estate to pay the debts and charges of a deceased person or of a person under guardianship or conservatorship, the adjudication of said court as to the existence of such debts and charges shall be final so far as the same may affect any title acquired by virtue of such license, but shall not affect the right of the executor, administrator, guardian or conservator to contest the validity of such debts and charges.