Is it permissible to buy goods from a grocery store at a price lower than their true value without the owner's knowledge?

If the grocery store belongs to your friend's father, and your friend disposes of its contents without his father's knowledge or permission, then this action is impermissible. This is because a son's disposition of his father's property is contingent upon his father's permission and willingness, based on the Prophet Muhammad's (peace be upon him) saying: "Indeed, your blood, your wealth, and your honor are sacred to you, like the sacredness of this day of yours... until the Day you meet Him... Behold, do not commit injustice... Indeed, the wealth of a Muslim is not permissible except with his willing consent," and his saying: "Everything of a Muslim is sacred to another Muslim: his blood, his wealth, and his honor," and his saying: "Whoever wrongfully seizes the property of a Muslim, he will meet Allah Almighty, and He will be angry with him."