Is it permissible, Sharia-wise, to appoint a third party – a neighbor – to carry out a currency exchange operation (Euro for Dinar) to fulfill the condition of taqabud (mutual possession), and is it permissible for the agent to return the amount that has been taken possession of to me as a trust after completing the operation, until the customer receives it?

There is no objection to the customer appointing your neighbor to sell the currency, as long as it is a genuine代理 that fulfills the Shariah conditions. There is also no objection to the agent depositing the amount received under the agency with the questioner, with the permission of the principal. The hand of the agent and the hand of the trustee are hands of trust; they are not liable except in cases of transgression or negligence. According to the Hanbalis, it is permissible for the customer to deposit the amount with the questioner and then appoint him to exchange it himself. In this case, the contract session is the time of the phone call.