Is it considered forbidden for a pharmacy to take an amount greater than what was agreed upon with the insurance company from the client, without his knowledge?

It is not permissible to take anything from the client in excess of the agreed-upon price except with his knowledge and consent. Otherwise, that would be consuming wealth unjustly, and it would be deception and a betrayal of trust. This is based on Allah's saying: "O you who have believed, do not consume one another's properties unjustly but only [in lawful] business by mutual consent." And His saying: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." And the Messenger of Allah, peace and blessings be upon him, said during the Farewell Pilgrimage: "Indeed, your blood, your wealth, and your honor are sacred to you, like the sanctity of this day of yours, in this month of yours, in this land of yours, until the Day you meet Him... Behold, do not oppress! Behold, do not oppress! Behold, do not oppress! Indeed, the wealth of a person is not lawful except with his willing consent." And his saying: "Whoever cheats is not one of us," and in another narration: "Whoever cheats us is not one of us, and trickery and deceit lead to the Fire."