Amazon customer is threatening me to return the product or i give him refund.

You have to refund the buyer, as an amazon customer has the right to ask for a refund.

If he wants to return then ask him to do so, as buyers normally do this, so if you will refund he will keep the product, if he returns the product and then gets refund then you will get the product which may be sellable so you will have no loss

You should have to refund if he is asking for that because Amazon if he returns the product and gave you a bad review then it will be trouble for you. usually, that has not happened but for the time being you have to refund.

It is normal for buyers to return products, so if you refund, he will keep the product, if he returns and gets a refund, you will have the product, which may be able to be sold, so you won’t lose anything.

Just go ahead and provide them a refund, that way you’ll have a better impression on them, if you won’t that will get you into trouble and you’ll lose a potential loyal customer.