Find any mission that has an advance payment. Start the mission and immediately abort it. After this is done, you will see your earnings displayed in two boxes. One box is for your new money amount that was earned and one for the deductions for repairs, ammunition, and the special deduction in the event that something was destroyed that was not supposed to. Note that there is no deduction, and you will get the amount that was promised to you in advance. Make sure to save your game before you do this, as the game will end if this is done too many times.