Bava Metzia 14 - Power of Attorney (Finds)

Suppose Reuven sold a field to Shimon with a guarantee (Reuven agreed to refund Shimon's money if the field is collected by Reuven's creditors). Reuven's creditor comes and attempts to collect the field from Shimon in payment of Reuven's obligation. In that case, the law is that Reuven may go and argue the case with the creditor to prevent him from collecting the field. And the creditor may not tell Reuven, "You have no 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.

Art: Good News by Hubert Salentin