Is it permissible to take the surplus from Ramadan iftar donations and send it to the poor in another country?

If the poor to whom the donations were sent are in dire need of them, then there is no blame upon you for changing the condition of the endower to what is more beneficial, because the endower's intention is to multiply the reward. Therefore, there is no sin upon you for what you did if it brings more reward to the donors, especially if those funds were surplus from the iftars.