Any sale or encumbrance to the fiduciary, his spouse, agent or attorney, or any corporation or trust in which he has a substantial beneficial interest, or any transaction which is affected by a substantial conflict of interest on the part of the fiduciary, is voidable by any person interested in the estate except one who has consented after fair disclosure, unless:
a. The will or a contract entered into by the decedent expressly authorized the transaction; or
b. The transaction is approved by the court after notice to interested persons.