Is the price difference of which the partner was unaware considered ill-gotten gain (haram) in a deal involving the purchase and sale of fabric with profit-sharing, and how can this be rectified?

The price difference belongs to the client, and you must return it to him, because keeping it is considered a betrayal of the trust that Allah has commanded to fulfill, and the Prophet, peace and blessings be upon him, made the betrayal of trust one of the signs of hypocrisy. You must repent to Allah, and there is no need to inform your friend; rather, it is sufficient to return the difference in any way that avoids embarrassment.