As a partner in a fruit shop, is it permissible for me to buy fruit from my brother, the merchant, at a price lower than the wholesale price, and keep the difference for myself, while I charge my partner the market price?

It is not permissible to take the price difference alone without the partner's permission, because an agent does not buy with his own money. There is no harm in showing favor to a brother in purchasing, but taking the price difference without the partner is a breach of trust. The Messenger of Allah, peace and blessings be upon him, said: "Indeed, Allah says: 'I am the third of two partners as long as one of them does not betray his companion. If he betrays him, I depart from between them.'" The meaning of "I depart from between them" is the removal of blessing and care.