But if conducting the trade himself is mentioned as a...

But if conducting the trade himself is mentioned as a condition [as opposed to a restriction] in the contract – for example, it is said, ‘I give you this money so that you trade with it on condition that you do it yourself’ – and the worker is unable to do so, the transaction is not void. However, the owner has the option (khiyār) to annul (faskh) the contract in case the worker does not conduct the trade himself.

Furthermore, if the contract mentions neither a restriction nor a condition but the worker is unable to trade even by appointing someone else, the contract is void. If he is able to trade at the beginning but not later, the contract is void from the time he cannot trade.