Complex sentence: The price was low and not realistic to accept for King Sigismund and it would be sure to be understand as putting pressure on Teutonic Knight to buy the property since they could not afford being surrounded by polish forces from the west.
Split into simpler sentences: The price was low and not realistic to accept for King Sigismund. But it would be surely to be understood to put pressure on the Teutonic Knights to buy the property since they could not afford to be surrounded by Polish forces from the west.