What is the ruling on selling an item back to the same seller for a lower price after having bought it from him and paid its full price, due to the seller's refusal to accept its return?

There is no objection to selling the item back to its owner who bought it from you, as it is a valid sale concluded by mutual consent between the two parties, and there is no religious prohibition against it. It would have been better for your friend to return the entire price to you and not take anything from it, based on the Prophet's (peace be upon him) saying: "Whoever rescinds a sale for a Muslim, Allah will forgive his sins." However, since the seller refused to rescind the sale unless it was a new transaction, and you agreed to that, and the sale was completed, there is no objection to it.