How transfer in consideration of debt, or, subject to future payment, etc., to be charged

Where any property is transferred to any person in consideration, wholly or in part, of any debt due to him, or subject either certainly or contingently to the payment or transfer of any money or stock, whether being or constituting a charge or encumbrance upon the property or not, such debt, money or stock is to be deemed the whole or part, as the case may be, of the consideration in respect where of the transfer is chargeable with ad valorem duty: Provided that nothing in this section shall apply to any such certificate of sale as is mentioned in Article 17 of the schedule. Explanation- In the case of a sale of property subject to a mortgage or other encumbrance, any unpaid mortgage money or money charged together with the interest, if any, due on the same, shall be deemed to be part of the consideration for the sale: Provided that, where property subject to a mortgage is transferred to the mortgagee, he shall be entitled to deduct from the duty payable on the transfer the amount of any duty already paid in respect of the mortgage.