Is it considered a permissible lie to inform two people that a third person sends his greetings to them, when the greeting was sent to only one of them, based on the assumption that the sender would have greeted them both had he known of their presence?

Lying is forbidden by Islamic law and leads to wickedness. However, it is possible to avoid lying when conveying greetings by saying, "So-and-so sends you their regards," intending to address the recipient with reverence, or by employing dissimulation (tawriyah), meaning one intends that the sender is sending greetings to the recipient as part of their greetings to the righteous servants of Allah during the tashahhud.