Sold a live demo phone

I recently bought a phone for my mother at MTN and after some time we realised the phone had a problem,so I called Microsoft mobile ***) ref # ***), and was advised that the phone was a live demo phone and Microsoft also advised that the phone was not supposed to be sold to me, it is ******* to sell that phone as it was designed for shop display, so I took the phone back to MTN and the first clerk i spoke to disputed what i told her,I asked her to call microsoft she said theres no need, the supervisor then checked the phone and confirmed it was a live demo, so they said they will send the phone for a warranty assessment (which doesnt make sense to me) but there was also a problem because I had lost the user manual, everything is there besides the user manual (like really??)will my query not be attended to because of a piece of paper?on a phone that I wasnt even supposed to be sold in the first place?and when the store called the repair centre the EMEI didnt even exist according to them which made me wonder as well do we really need to keep fighting about bad service??All i want is for this to be resolved.