And next tonight we have a consumer alert for car buyers. You think the deal is done on your car and suddenly after you've gone home, you have to pay more money. It's called the yo-yo sale and abc's... See More
And next tonight we have a consumer alert for car buyers. You think the deal is done on your car and suddenly after you've gone home, you have to pay more money. It's called the yo-yo sale and abc's chief investigative correspondent brian ross shows us how it works and how to fight back. Reporter: When miss smith left the car dealership with her pt cruiser she thought the car was hers. Once I signed the contract and they signed it, the deal was done. Reporter: But a week later, the dealer told her the deal was off unless she put down $300 more. So she went to the dealer wearing a hidden camera for us. So we can do a new contract, a new deal for you and it's going to be through here. Reporter: It's a practice often compared to the classic children's toy, the yo-yo. According to people at the federal trade commission. Yo-yo might sound like it's making light of the term but it really is kind of showing you the whole jerking the consumer back and forth issue that we're concerned about. Reporter: The car dealer industry group says the vast majority of car deals occur without mistakes or misconduct. When we showed up at the dealer with miss smith, the salesman she had she was doing wrong, that she had signed a document promising to bring the car back if the financing did not come through. Can you show it to me? I don't need to show anything to you. Reporter: He refused again and again to show it to me until the owner showed up and made him turn it over. Give it to him. I don't care. I've got nothing to hide. Reporter: Then the truth came out. She didn't sign this. It's signed by you. That's why you didn't want to show it to me. Reporter: Vindication for miss smith and a valuable lesson to anyone buying a car. You can see more of brian's reporting tonight on abc's "the lookout" at 10:00 p.M. Eastern.
This transcript has been automatically generated and may not be 100% accurate.