I think Williamson is in high stress after the robbery. Then, he has do deal with the police, and Roma cursing and yelling at him.
He might also feel guilty because he was supposed to come back yesterday night to file these contracts as he always do but this one time, he didn't as he stayed home with his kids. He probably wants to protect him since it could be suspicious that a robbery happened at the very same moment where he didn't came back at the office.
Later, when he confronts Levene:
I'm saying this, Shelly... usually I take the contracts to the bank, last night I didn't. Last night I stayed home with my kids.
How did you know that? One night in a year,I left a contract on my desk. No one knew that but you.
His job is not in danger as he is not a salesman, so he might not care about this specific deal and the Cadillac, so think he just wants to calm Roma down to get rid of him at this specific moment.