Per diem as contractor vs fed

I did 6 years with FS and just recently got back into fire with a contractor as a medic. I just got an RO for a fire and per diem/lodging is authorized. I asked my boss if the per diem is just added to my paycheck and he said if I don’t spend it I don’t get it? I recall that we would get our per diem tax free through FS travel even if we didn’t spend it on a travel card or whatever. Can a contractor even do that? He’s gonna get the per diem from the fire either way