What is the ruling on purchasing vouchers worth 120 Euros for 20 Euros each, which can be used to buy products from a company that sells phones, and is this considered usury (riba)?

There is no harm in purchasing company vouchers that represent offers for its services and benefits. This is not considered a purchase of currency, which would prohibit differentiation [in value]. Rather, it is the purchase of an offer, and it is permissible [to acquire it] for its equivalent value, or more, or less.