Is a sale void if the purchased portion is claimed by another owner (mustahaq)?

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When a portion (shaqas) is purchased in exchange for a slave or a specified price, and it is found that the portion belongs to someone else (mustahaq), the sale is void. The right of preemption (shuf'ah) is not established in this void contract because preemption is only established in a valid contract that transfers ownership to the buyer; a void contract is treated as non-existent.