The bottom line here is that in making your purchase decision, you were either acting in your client's best interest, or you were acting in your own best interest.

If you claim that you were acting in your client's best interest because he would have made the same purchase decision because of the rebate, then you are correct. You were acting in his best interest. However, that also means that the rebate is HIS.

If you buy that stove, but keep the rebate for yourself, then it's hard for us not to believe that the rebate didn't steer your decision making right from the start. In that case, you were acting in your own best interest, and betrayed your client's trust.

Give the rebate money to your client and get a tip and some word of mouth advertising out of the deal. If your client finds out there was a rebate paid on that purchase and he never saw a penny of it, he'll believed you betrayed his trust. If you tell him about the rebate, and offer him half, he'll believe you betrayed him because if you were acting in his best interest, he should get the entire rebate. So, he'll badmouth you for the rest of his life.

If he relates the story exactly as you related it to us, his audience will agree that ethically, you have an obligation to return the rebate to him because you were acting on his behalf in making the decision to purchase the stove which offered the rebate. You can't say you were truly doing that if you gave him the stove, but kept the rebate for yourself since the rebate was the reason for purchasing THAT stove. Or, rather, you can say that, but not too many people would believe you.