Sent my 12 year old for vegetables with ₹500. He came back with change and a pineapple

First time sending him alone to the vegetable vendors near our building. Eight things on the list, ₹500, phone in his pocket. He came back with seven of the eight, a pineapple nobody asked for, ₹60 change and a full report on which aunty gave him extra coriander. The pineapple was "a good deal". I'd like to make this weekly and slowly hand over more, maybe the monthly provisions eventually. How did you build up money responsibility at this age without it becoming a lecture from me every time the maths doesn't add up?