Did the food from the coupon offer come in a lesser amount than what you were expecting?

There is offer in a restaurant in which if we buy food of Rs 25.00 we get a meal of the mentioned choice free. I had sent my servant with tat coupon to get the offered food for free. He had his friend working there so instead of buying of Rs 25 he bought it food worth of Rs 15 and used that coupon to get extra meal for free. The restaurant had clearly mentioned that if we buy food worth only for Rs 25 or more we can use that coupon to get free meal. So after eating it I realized that I had done wrong I do not know what to do I cannot inform those restaurant people as it is not possible to do? So next time I bought food worth of Rs 55 instead of Rs 25 to get a free meal so that it could tally. Is it okay or not please help me because I do not want to consume anything haram in my stomach please give a solution for it.