If a cohen cannot go to a cemetery because he would become tomei, does the mitvah of returning a lost animal override that negative, does a postive cancel a negative commandment. woud this hold true in a house if there was a dead sheret in it, or a dead body. cohen finds a lost animal is he obligated to return if he is tome.
Dear Jeffrey,
The Gemara explains that even though a positive cancels a negative commandment, here the positive (returning the lost item) cannot allow breaking a positive and negative commandment together of a Kohen (Be holy and don't become Tamei).
A house with a dead body is the same situation.However there is no sin for a Kohen to become Tamei because of a Sheretz (only he then cannot eat Truma and Kodshim and can't enter into the Mikdash).
If the Kohen is already Tamei he is still forbidden to make himself Tamei again (some say only on a D'Rabanan level), so the same Halacha applies.
All the best,
Reuven Weiner