NOTE: This involves editing a game file; create a back up copy before going any further.

Use a text editor to edit the *.xml file that corresponds to your saved game name in the directory /john deere american farmer/data/games

The first line in the XML file should resemble <money>300000</money> where the number is the amount of money you have. Changing that value will change the money you have in the game. Save the file as XML (not as text) and the effects should be apparent.