I'm guessing that the contractor cashing the checks is actually a sub-contractor cashing checks from the general contractor's account at the original poster's bank. Unless you feel the checks are structured somehow, I don't think I would be jumping to conclusions about what the sub-contractor is doing as far as payroll taxes goes. It could be that he gets paid in stages for a job - $5,000 when construction reaches a certain stage, $4,000 when the next stage is complete, etc. He could be cashing the checks so that he avoids a hold being placed by his bank if he deposits a check drawn off your bank or he may just want to make sure he doesn't get stiffed or stung by an NSF or stop payment. I think, absent any more solid information, I would not file a SAR. Even if he is paying his employees in cash, it does not necessarily mean that he is not doing proper withholding or that perhaps the workers are independent contractors themselves. If all else fails, call your customer and the guy cashing the checks and ask them directly what is going on.
_________________________
Knowledge is knowing what to say. Wisdom is knowing when to say it.