While playing the game, press [Ctrl] + [Alt] + [Shift] + [End].
Save the game immediately before completing a mission. Complete the mission as usual, and save the game in a different save position. Reload save that was made before the mission was completed. Complete the mission again. The statistics screen will combine the money from both missions. Repeat this to earn as much as needed.
Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "dark.cfg" file in the game directory. Change the value after the "cash_bonus" entry to increase the amount of additional money available at the start of each level.