Is it ethical to ask a customer for a favor in return for giving them a discount?

definitely not,,as simple as that

ethically wrong to ask any type of favor. Business can't afford such act.   

Not at all ethical to ask for a favour from a customer and in return offering a discount. This will certainly damage the reputation and image of the organisation and lacks professionalism 

It is not ethical at all to ask for a return for any favor that you are doing in your life but some customers usually offering to pay the favor back and it is smart to have it without affecting your job or your organization policy and procedure and at the same time to satisfy the customer needs .

Certainly not ! Discount offer is purely to move the business forward commercially.