Grab your opponent and interrogate him until he drops a credit card or his wallet. Then, execute him and choose to steal. He will either give you a credit card or wallet, depending on what you got the first time. Normally when you kill an enemy and steal from them, you will only get either a wallet or a credit card, and never both.

Use a close range head shot to get $5,000 or more in credit cards or money, jewelry, and armor depending whether you need it or not.