Is a mandate valid if the agent is authorized to buy anything without specifying a price limit?

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A mandate is invalid if one says, 'Buy for me whatever you wish,' because the agent might purchase something for which the principal cannot afford the price. However, a narration attributed to Ahmad suggests validity, referencing a case where two partners agreed that whatever one bought would be shared between them, which the Imam approved of. By analogy with partnership (shiraka) and investment management (mudharaba), where the agent has the authority to buy, the agent may only purchase items at the customary market price (thaman al-mithl) or less, must not buy what the principal cannot afford, and must not buy what the principal does not see as beneficial.