Dishonest Branch

I would like to report how dishonest this branch is, last week they had an opening special which i made sure i was there in time however, the branch did not have any stock in their store but had not sold the qty so the staff member decided to still go ahead and write a receipt for me and advised that i should come back in a week with it when they receive their stock to collect. I asked and i stress this several times, should i pay for this and he had said \ NO -you pay when you collect the item. "Did i know any better. So i go into the store this afternoon and they said that the note they had given is not valid and i need some sort of a voucher called a 'call back voucher\"" to claim for the product at that price. Their staff member did not inform me that i need to pay or i need this voucher. I do not know how their processes work and if they do not inform me"