What is the Shar'i (Islamic legal) response to someone who seizes another's goods, claiming it is his right, and refuses to return them despite knowing it is usurped property?

If a speculator loses another's money due to his negligence, he is liable to the owners for their funds, and this money becomes a debt upon him. If the debtor's money is insufficient to pay his debts, the existing money is divided among the creditors proportionally to their debts, and the remainder remains the debtor's liability. No single creditor is permitted to monopolize the money to the exclusion of the other creditors. In case of dispute, in the absence of a Sharia (definition) court, the parties should agree upon a Sharia ruling to arbitrate between them.