If you want my advice don't give them a dime until the job is complete.
Most contractor horror stories involve paying before the job is done,
that includes partial payment. I fronted one guy $200 so he could get
some front tires for his truck, he bought the cheapest ones available
and he and his crew smelled of alcohol the rest of the week. I was
probably lucky, at least they showed up, but some of the lines in the
roof aren't as straight as they might have been. As to materials, part
of the act of becoming a contractor, as opposed to a workman, is to
have the resources or lines of credit to buy the materials up front.

If a contractor needs $ for truck tires stay well away from him, IMO