You must still pay the 10% interest fee to the Bank immediately. If you choose to lift the mortgage later in the game, you will be required to pay the mortgage value plus another 10% interest to the Bank. Key Strategic Constraints
When you acquire a mortgaged property through a trade or sale, you have two immediate options regarding the mortgage: buying a mortgaged property in monopoly
You cannot build houses or hotels on any property in a color group if even one property in that group remains mortgaged. You must still pay the 10% interest fee