The police will fine you for various things such as partying late at night and using the chemistry set too often. However, if you do not have enough money to pay the fine, nothing will happen. To avoid fines, send one of your Sims to greet the police officer, then quickly enter buy mode and buy something that takes your total cash to just below enough to pay the fine. The police officer will announce that you are being fined, then leave without collecting it (since you cannot afford it). After that, sell the item you bought. It should still sell for full price, since you just bought it a moment ago, to keep all your money.