Whoever at Mephisto (shoes) decided that it is a company rule to refuse to reimburse the difference when exchanging shoes should look for another job. Even the AI that will replace them would ‘know’ that this is an anti-customer move.
There’s no excuse for this abusive policy. Nothing prevents you from simply crediting the card used for the purchase, or giving back the difference in cash. Forcing a longtime customer to return within a year to buy more stuff is off-putting. It makes the customer want to never shop at your store again. That’s exactly what will happen when I find a way to spend the 30€ you owe me.