Is a contract valid if someone is hired to purchase specific, designated garments?

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There is a possibility that the contract is invalid because purchasing specific items can only be done by one individual, and if that seller refuses to sell, achieving the contracted action becomes seemingly impossible. This differs from the sale of items, which is more readily achievable. If the hired agent accomplishes any part of the intended action, they earn the fee; otherwise, the contract of hire is nullified, which is the same ruling applied when neither the seller nor the buyer is specified.