How can one respond to Christians' greetings for their holidays, such as "Kul 'am wa anta bi-khair" (May you be well every year), without falling into what is religiously prohibited, while maintaining social relations in a work environment?

Congratulating disbelievers on their religious holidays, such as Christmas, is forbidden by consensus. It is also impermissible to return their greetings for these holidays, as they are not holidays for Muslims, nor are they pleasing to Allah. Furthermore, these holidays may be innovations in their religion or abrogated by Islam.