What is the ruling when a usurped item is mixed with something worthless, such as oil mixed with water?

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If the usurped item is mixed with something valueless, like oil mixed with water or milk mixed with water, and separation is possible, it must be separated, and any resulting deficiency must be compensated for. If separation is impossible or would spoil the item, the usurper must return the equivalent. If separation does not spoil the item, he returns the item and compensates for any deficiency. If separating the item requires an expenditure, that cost is incumbent upon the usurper because it resulted from his action.