Just before completing the mission, do a save or quicksave. Complete the mission and save it in a different save slot. Reload your last save or quicksave before completing the mission. Complete the mission again. The statistics screen should now have ADDED TOGETHER the loot from BOTH mission completions. Apparently, the variable which stores treasure amount at the end of the mission is not being cleared between loads. You can keep doing this until you have all the cash you need to buy out the store at the loadout screen for the mission immediately following.