You're allowed to calculate the return using percentages, but that doesn't make it interest/riba. Halal loans are collateral-backed and you pay them a fee for the time they hold your collateral. So you sell them a house worth €100k and they give you €120k and you have to pay them 10% more to get it back, so €132k. They hold the title for the duration of the loan, with you retaining a right to repurchase. That is not interest (money charged for a rate for the time you hold it).