Is it possible for you to get h.i.v by sharing boxers with precum?

Unlikely: First i wonder why you are sharing boxers in the first place. The longer the time between wearer #1 and wearer #2, the most HIV will die assuming wearer #1 has a viral load (if not then there is no worry at all). Wearer #2 would then have to have an open wound or scratch in contact with the precum for HIV to spread. All in all a low chance - boxers are cheap: get your own.