What should a person do who deposited $10,0 with a friend to establish a company, then they disagreed on the deposited amount, as the friend claims it was only $7,900, knowing that the friend paid zakat on $10,0 and refuses to acknowledge the shortfall, while the depositor fears accusing the friend's wife, in whom the latter trusts?

The trustee is not liable for what is lost from the trust without exceeding limits or negligence on his part. If it is lost due to his negligence, then he is liable. The amount you ordered to be spent as Zakat from your money is incumbent upon you, and he may take it from your trust with him. You may reclaim the excess amount from the one to whom you paid the Zakat, as he is not entitled to it. You may also intend the excess amount as a voluntary charity.