Most of us know the situation: We are receiving a letter delivered by the postman and suddenly we notice that the post office forgot to put a postmark over the stamp. Are we then allowed to re - use the stamp ?
Halacha tells us not to re - use the stamp, as this would be GNIVAH (Theft). The first person paying for the stamp and sending out the letter was paying for the delivery service carried out by the post office. Thus the post office received the money and sent out the postman. Meaning, the service is paid for and if you want to send your own personal letter by re - using the stamp, you would be stealing from the post office. Therefore it is not allowed to re - use such a stamp.