When he is walking in the woods with the devil in the first place, there is no indication that Tom refused the devil right then and there, but rather wanted to ponder it for a while, to see if he wanted to take him up on his offer. The text states that when Old Scratch stated his terms, "They must have been very hard, for Tom required time to think of them, and he was not a man to stick at trifles where money was in view." So, they were hard conditions, and Tom wanted to think about it. He asks for proof of the devil's word, which the devil gives as a permanent finger-mark in Tom's forehead.
It is after he gets home and tells his wife about the incident that we get Tom's initial, flat-out no, and the reason for it was that "However Tom might have felt disposed to sell himself to the devil, he was determined not to do so to oblige his wife; so he flatly refused out of the mere spirit of contradiction." He refuses, just because his wife wants him to say yes. They have a contentious and bitter relationship, and she really, really wants the money the devil is offering. So, just to spite her, Tom refuses. Kind-of a funny and silly reason, but it fits their relationship. Eventually, after his wife is out of the picture, he does accept the deal, and lives the rest of his life-and afterlife-with Old Scratch.