This

Il a acheté pas une pomme, mais une orange.

does not work in French. You would have to say:

Il n’a pas acheté de pomme, mais une orange.

Or:

Il a acheté non pas une pomme, mais une orange.

Which is correct, depending on the context. Before that sentence, we would need something that built up the expectation that he would buy an apple, to justify the “non pas”. Outside of that context, I would not use “non pas” and fall back onto the previous example (“Il n’a pas acheté de pomme, mais une orange.”)