728. Proof of principal mortgage debt.

Where a mortgage is given for a specific sum stated to be then advanced, the receipt of which is acknowledged by the mortgagor, the mortgage deed is prima facie evidence of the amount of the advance1, and, accordingly, the principal debt is proved by production of the deed, with the receipt contained in the deed or indorsed on it2. The receipt is not conclusive, however, and the mortgagor is entitled to show by oral evidence that the sum named was not in fact advanced