[Name withheld] wrote via the Internet:
Is it permissible to re-use U. S. postage stamps that came through un-cancelled? What if the use is for sending charity?
Dear [Name Withheld],
A postage stamp can be thought of as proof that delivery of the letter has been paid for. If so, reusing a stamp is tricking the Postal Service into thinking the letter has been paid for, which in fact is not the case. This would be called 'genevat daat'- fraud - something that's certainly forbidden.
You suggested that perhaps the stamp could be used for sending money to charity. Sorry, but giving charity is not a proper way to launder stolen money.
I once heard this idea expressed by my teacher, Rabbi Shmuel Feivelson, shlita. He asked, "Why does the section of the Torah dealing with monetary laws (Parshat Mishpatim) precede the commandment to donate to the building of the Holy Sanctuary (Parshat Terumah)? Because, before you donate money to charity, you have to make sure it's rightfully yours."
- Shulchan Aruch Choshen Mishpat 348:2.