Mathews comments at the end that Gus died of a heart attack, implying that it was not a murder. Since the heart attack was brought on by the assault and battery committed by the con man, he would be liable for a homicide charge of some sort. This would likely be manslaughter, which is a lesser charge than murder due to lack of intent to kill.
Mathews had no cause or warrant to search the briefcase nor to demand that the waitress open it. As such, any evidence seized would be inadmissible in a trial.
At one point while conversing with Mike Purdum, Mattie addresses him as "Gus".