You would pay the rate you agreed upon with the owner of the receiving account. There is no pre-defined exchange rate for such transactions, so you would need to negotiate one with the holder of the US bank account.
On the other hand, if you have jumped through all of the bureaucratic hoops for transferring ARS out of the country via PayPal and merely send the owner of the other account a payment in ARS, then PayPal will convert the ARS to USD using the official rate (minus PayPal's fees).