Is it permissible for a doctor to delay breaking his fast if he is performing surgery?

The Sunnah is to hasten breaking the fast immediately upon sunset. The Ummah will remain in prosperity as long as they uphold this Sunnah. The wisdom behind this is to prevent the day from being extended into the night, and because it is more comfortable for the fasting person and strengthens them for worship, and it is a prompt تناول (consumption) of what Allah has made lawful. The Sunnah is to break the fast with fresh dates (rutab); if not available, then dried dates (tamr); if not available, then water; and if none of these are available, then with whatever food or drink is readily available. If the fasting person finds nothing to break their fast with, they break it by intention.