Could you check the use of the word 'flaw' and 'if someting comes up' in this dialog?!

Customer: If there's a flaw in this CD player can I have it replaced?
Salesperson: Yeah! If something comes up, you can always bring it back together with the receipt and we can have it replaced or you can have your money back.

Does the dialog sound normal?

I usually use flaw for human attributes (e.g. All of us have flaws.). I use "something came up" for an undesirable incident (e.g. Something came up. So, I couldn't join you guys.).

What you described as your usual use of 'flaw' and 'something came up' is perfectly fine, however, the dialogue does sound normal Emotion: smile The tone of voice is quite informal in this case here.

Remember that in English a work or phrase can be used in more than one situation! Emotion: smile

'Flaw', yes. 'Come up', no. 'Come up' = casually occur. Use 'Certainly! If something goes wrong / If something is wrong with it / If it's broken, you can always bring it back...'
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Thank you very much.
I really felt there was something wrong with the tone.
John: In case there's a flaw in this CD player, can I return it?
Cashier: Yes. But you must return it within 30 days.
John: Is there a charge for that. I know other stores have a restocking fee.
Cashier: There will be no charge at all. However, you must show us the receipt. Could you sign here please?!

John: Thank you.
Cahsier: All right. Keep your receipt. If something comes up, you can show it to us and we can gve you a refund.
John: Thanks I'll put it in a safe place.

Does it still sound unnatural?

Thanks. Just wanted to double-check.
Yes, is still sounds odd to me. If something comes up = if there's a casual occurrence of something. 'Casual' is the wrong implication in this situation.

Some other parts of the dialog are not particularly natural, either. Most people would say 'if there's something wrong with', not 'if there's a flaw in', I think. 'Must' too is not so common in speech. And why would someone mention putting a CD player 'in a safe place'? That too sounds a little odd to me.

And yes: over = again.
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Thank you very much MM!