If it really was a check, it must be drawn from some financial institution. Force bankers to testify. Follow the money.
It was a fake check. The cops arrested Haldeman when he tried to deposit it, as it was apparently the "evidence" they wanted to establish intent to commit a crime (blackmail, not fraud).
But, I'm still skeptical. The prosecution is going to have to prove that Haldeman intended to harm Letterman if he didn't buy the "screenplay". Unless he made the threat explicitly, I'm not sure that charge will stick.
Haldeman did do one thing wrong: he reportedly "contacted" Letterman by leaving a package on/in his car. He should have sent it FedEx, if he wanted it to look legit.