What is the justification for allowing the usurper to reclaim substitute payment if the usurped juice turns into vinegar?

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The vinegar is considered the same entity (ayn) as the original juice, having only undergone a change in attribute. Since the usurper has returned the modified item, he has the right to reclaim any substitute payment made for the original item. This is analogous to a situation where an item is usurped, then usurped from the first usurper and subsequently returned to the original owner, or when usurped fetus in the womb (haml) becomes a ram (kabsh); the original item's transformation does not negate the right to recover the initial substitute payment upon the return of the modified entity.