For an SP101 in 9mm in excellent condition for under $300 ... yeah, I'd jump on that deal. I probably would have paid $400 on the spot just for the uniqueness of that caliber in an SP. I've never really looked for a 9mm revolver, but I really like the SP101 and wouldn't hesitate at that price.
You'd better take him up on that on Monday before he changes his mind or finds another buyer. Better yet, call him tonight and have him bring it to work on Monday with your name on it.