Weigh Cost Before Denying Accommodation Request

Q. An employee we hired a couple weeks ago through a temporary agency (he is on their payroll) has just told us that he is Muslim and can't work on Fridays. During the interview, he was asked whether anything would prohibit his working a proposed schedule that specifically included Fridays for the next 30 days. He said no, and this is in writing. He did not mention this problem until he had already been working for a week or two. Can we let this guy go? —M.R., South Carolina