If it's an upmarket supermarket which uses discounts and vouchers as part of their marketing, tell the friend you have a discount but only on a large purchase size, e.g. 15% off a 50 spend or more, assuming 50 of your currency is a large supermarket purchase in your locality. Tell them that they may as well come to the supermarket so you can use your large voucher, as you can't spend that much yourself. At the checkout, you may need to deceive your friend that you e.g. lost the voucher, however that you'll cover their 15% off.
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