(c) Where the mortgagor ostensibly sells the mortgaged property- on condition that on default of payment of the mortgage-money on a certain date the sale shall become absolute, or on condition that on such payment being made the sale shall become void, or on condition that on such payment being made the buyer shall transfer the property to the seller, the transaction is called a mortgage by conditional sale and the mortgagee a mortgagee by conditional sale: Provided that no such transaction shall be deemed to be a mortgage, unless the condition is embodied in the document which effects or purports to effect the sale;