It is the duty of the trustee to recover, for the benefit of the estate, property which was conveyed by the grantor or assignor in fraud of his creditors, or which was conveyed or transferred by the grantor or assignor for the purpose of giving a preference. A preference, under this section, shall be deemed to have been given when property has been transferred or conveyed within four months next preceding the registration of the deed of trust or deed of assignment in consideration of the payment of a pre-existing debt, when the grantee or transferee of such property knows or has reasonable ground? to believe that the grantor or assignor was insolvent at the time of making such conveyance or transfer. (1909, c. 918, s. 2; C.S., s. 1611.)