Is it permissible to take an additional sum from a craftsman in exchange for completing a task, such that this amount is above his original wage?

If a person is commissioned to bring a craftsman to contract with him to accomplish a task, he is considered an agent, and he must work for the benefit of his principal faithfully. If he contracts with the craftsman for a price, and informs the commissioning party of a higher price, this is deception and a breach of trust, which is forbidden by Islamic law.

Allah Almighty says: (Indeed, Allah commands you to render trusts to whom they are due).

The Prophet, peace and blessings be upon him, said: (Render the trust to the one who entrusted you, and do not betray the one who betrayed you).

The Prophet, peace and blessings be upon him, also made the betrayal of trust one of the signs of hypocrisy, saying: (The signs of a hypocrite are three: whenever he speaks, he lies; whenever he promises, he breaks his promise; and whenever he is entrusted, he betrays).

And the Prophet, peace and blessings be upon him, said: (Whoever deceives is not of us).