Does placing two pieces of fruit in the shop suffice for previously damaging a single piece, knowing that I do not know its old price and whether it is more expensive than the price of the two pieces now?

Whoever damages something from people's property, whether intentionally or by mistake, small or large, is obligated to guarantee it; for intentional and unintentional damage to property are the same. The fact that the damage occurred before puberty does not waive the guarantee, rather it only waives the sin, and the one who caused the damage, or more than one, must return it to clear their conscience.