Well, there are no cheats to decrease money - only cheats to increase money. However, one way you could decrease the amount of money you have on The Sims 2 is to just buy stuff. Try buying really expensive paintings and hanging them on the wall. That way, you decrease your cash but then you won't have all this junk just laying around your Sims' home and getting in the way. The paintings will fade away when you place them on the walls and you will only be able to see them if you choose to raise all the walls in your Sim home under the House Views.
Then you can put them on your sim's backpack and gift them to some sim else