This involves editing the careerSavegame.xml file. Make a copy/backup of the file before editing, and make sure you have a Career savegame.
Use notepad and go to C:\Users\user name\Documents\My Games\FarmingSimulator2011\savegame1\careerSavegame.xml
Look for the line that says money="5109236".
Enter whatever amount you want, and then save it.
Start game, and load your career save game.
(supplied by: FirebAll1)