Max Gold Cheat
Insert a CodeBreaker device into the Gameboy Advance's memory slot and turn on the game. The CodeBreaker console will pop up. Insert the code "0000E882 000A 100D1BFB 0007" and "820068B4 E0FF 820068B6 05F5." Turn off the console, allow the game to fully load and select any saved file to play. Now turn the CodeBreaker console on and press "Select" to access the Farm Earnings screen. Exit the screen again--this activates the codes. According to CheatingDome.com, this maxes out the total amount of gold in the game.
The Profit Cheat
Play the game long enough until it reaches "Year 5." When it reaches "Fall 10" on the calendar, visit the Peak of Mother's Hill between 6 o'clock in the evening to midnight. Once reaching the hill, an event called the Shooting Star Event will activate. When prompted to make a wish, select the third option. The day will automatically end. On the next day, ship as much produce and items as possible until 5 o'clock in the evening. Zack will visit afterward and double your money for that day.
The Truffle Cheat
Go to the church and give a mushroom to Carter. Continue giving him a mushroom every day until he becomes your friend, which may take up to a season. Now wait for fall to approach. When fall approaches, go to the church and enter the locked door behind it. If Carter's friendship rating is high enough, the door will be unlocked. Pick any truffles off the ground and sell them to a vendor--they sell for 500 gold.