Bava Metzia 14 - Power of Attorney
If Reuven sold a field to Shimon with a guarantee (Reuven agreed to refund Shimon's money in the event the field is collected by Reuven's creditors), and Reuven's creditor comes and attempts to collect the field from Shimon in payment of Reuven's obligation, the law is that Reuven may go and argue the case with the creditor to prevent him to collect the field. And the creditor may not tell Reuven, "You have not standing in the matter."
But otherwise, anyone who has no standing cannot argue the case in court, and attorney representation is disallowed, and so decides Shulchan Aruch. Other opinions, however, allow power of attorney.
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