Discount on cost of    laundry if one pays in advance
Question

Some laundry services tell people, buy a voucher for seventy riyaals for example and we will do laundry for you worth one hundred riyaals. If the customers do not buy this voucher, they will pay one hundred riyaals for one hundred riyaals worth of laundry; if they buy it for seventy riyaals, it will entitle them to have one hundred riyaals worth of laundry washed. So this voucher is a kind of discount. Is it permissible to buy it from them and use it for one's laundry?
Praise be to Allah.We put this question to Shaykh Muhammad ibn Saalih al-Uthaymeen, may Allah preserve him, who answered as follows:
Yes, it is permissible, because they are saying, if you buy the voucher in advance, we will wash your garment for seventy riyaals, otherwise it will cost you one hundred.
And Allah knows best.