I don't think you can. I would have to replay to get the exact wording, but when you beat the mega bird whose name I forget and bring the edelweiss flower to the granny, she thanks you and says that she appreciates your valiant efforts, as she knows you had to kill the bird, since the merchant left town. And indeed, the merchant is gone.
I would have to replay and try to buy the flower first, and then see if it's still possible to go and pick up the flower on the mountain, but doing those steps in reverse (first mountain flower, then buying from merchant) is impossible.
Maybe
@Maelion can shed more light into this?