What is the ruling on a teacher taking his full salary without deduction for lectures he had agreed to deliver before giving them, and then delivered them subsequently? And what are the implications if he had already taken the full salary?

The teacher's signing off on lectures that have not yet been delivered is forbidden (haram) because it involves lying. He must repent and return any money he took in excess of what he deserved. Allah Almighty says: "And do not consume your properties among yourselves through falsehood."