Revenue

Hello Mike I was thinking about a situation and I have a question. Its not directly related to F7 but I think you are the best person I should ask.....I hope you don't mind. Here goes: In a situation where entity A sells shoes worth 100 in the last month (December) of their financial year and gives a discount voucher of 20 to the customers to be used within six months. How much revenue is recorded for that 1 pair of shoes, is it 100 or 80? I was thinking that it should be 100 since we don't know exactly how many people would use the voucher but then I was also thinking to be prudent and not overstate revenue for that financial year we should use 80.