24. 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 whereof 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 No. 18 of Schedule 1. Explanation-In the case of a sale of property subject to a mortgagee or other encumbrance, any unpaid mortgagee-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 mortgagee 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 mortgagee. Illustrations (1) A owes B Taka 1,000. A sells a property to B, the consideration being Taka 500 and the release of the previous debt of Taka 1,000. Stamp duty is payable on Taka 1,500. (2) A sells a property to B for Taka 500 which is subject to a mortgagee to C for Taka 1,000 and unpaid interest Taka 200. Stamp-duty is payable on Taka 1,700. (3) A mortgagees a house of the value of Taka 10,000 to B for Taka 5,000. B afterwards, buys the house from A. Stamp duty is payable on Taka 10,000 less the amount of stamp-duty already paid for the mortgagee.