okay, I gotta ask, "what is several dollars more than the going rate?"
and just what kind of construction business is this that you have to have a CDL + HAZMAT?
and where are you located?
I had a boss once...he was so tight he squeaked. He was looking to cut corners every chance he got, and in the process violate regs.
Anywhooo...he relayed a story to me once about how he had a coffee cup filled quarters sitting on his desk. on top he had say like 4, one dollar bills.
then he noticed one day that two or three of the dollar bills were missing.
so he calls out one of his employees about it, figures it is him, and then fires him.
now this is the kicker...that has me still shaking my head after so many years working for the guy....the boss said, "I didn't fire him because he stole from me. I fired him because he was dumb about it. He should have just taken the money in quarters instead and I would have never noticed it."

a few years ago, I noticed the old boss's equipment up for sale on Craig's List. He had gone out of business. I had a good laugh when I saw that.
Anywhoooo....since this forum seems so hip on protecting the 2nd Amendment....how about a little love for the 6th Amendment....ya, know, the one that says you have the right to face your accusers/witnesses.
Have a meeting with him inside your office in private and show him the statement from the gas card, and tell him what you think happened.
See what his reaction is. Within 30 seconds you'll know if it was done maliciously with intent to steal/defraud.
And you'll know at least it was done man to man.