You may think it's the car salesman that's out to get you, but the real scams happen here. Trust me, I am a Finance Manager and have seen many other Finance Managers uses scams, tricks and ploys to separate nice people like you from their hard earned money.
The best advice I can give you is to never sign any contracts or purchase orders blank. Let me say that one more time, because I've seen it happen so, so, so many times...Do Not Sign Blank Contracts!
In addition, be sure you read (don't just take the Finance Managers word for it) everything contractual that you are being asked to sign and verify that the numbers are the same as what you agreed to.
This is most important with the contract and the purchase order. If the Finance Manager is moving too fast on these two, then ask them to slow down or repeat what it is you are confused about.