forget about the money. toss the letter. deny everything. they probably don't care about you. You'll be fine.
BUT! i'm gonna have to tell this story:
I knew this guy from work. Gratefull dead t-shirt, long hair kinda guy. Lives on the edge of town with his wife and his cute kid. One day, someone breaks into his house and steals his walkman, camera, and other junk. He forgets about it.
Then, 4 mo later. The cops come to his house with a warrant and bust him for possession of pot. Which any fool coulda told you he had.
This is what happened. Cops get a tip. They go to some teenagers house and find a bunch of stolen junk from the neighborhood. They press the kid, he gives up his friends, all the loot gets recovered. But, in with the loot is some pot. "Where did you get that?" the cops say.
Guess whos freezer it came from.
The moral of the story is: You'd probably better clean out your house for a couple of months. You'll be safer once the court-case with the source is closed.