It was the trick with the candle, wasn't it? 

That oddness aside, taking the merchant's offer would effectively make him miss his appointment with the Thayan historian. Don't get me wrong, he has serious misgivings about dealing with wizards in the first place, so it's probably not a total loss. Money is money, after all, and supplies will be necessary when he figures out where he's going next. If he's going anywhere for a bit.