Company A can only give a power of attorney to its own agent, it cant appoint an attorney for company B unless company A is the parent and company B a subsidiary. It does get complicated depednig on the rules of each company.
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If you enter an agreement to finnance a business and the other person gives you a vehicle log book and during the transection you find that the goods the person was to deliver were practically not possible to have all of them. How can now one proceed.