Is a transaction considered valid, or is it usury, when it involves purchasing a product for a friend with dollars, with payment to be made later in local currency, provided that the buyer first receives the product and then delivers it to the friend in exchange for the payment of an amount at a reduced exchange rate willingly set by the buyer?

This transaction is impermissible because it is a loan in dollars to be repaid in local currency, which is prohibited unless both parties agree on the exchange rate on the day of repayment. It is also not permissible to combine a loan with an exchange (mu'awadah). The correct method is to purchase the product by way of a promise to buy, and then conclude the sales contract after the product comes into your possession.