Negative Mitzvah 107
We are forbidden to change the type of sacrifice that was originally intended
Leviticus 27:26 "No one shall sanctify it"
Once an animal has been chosen to be offered as one type of sacrifice, it cannot be switched and offered as another.
For example, an animal designated as a peace-offering may not be then offered as a sin-offering.
After a person sets aside the animal for a specific purpose, he cannot change his mind and decide to bring it as a different type of sacrifice.
Positive Mitzvah 107
Impurity of coming in contact with a Dead Body
Numbers 19:11 "He that touches the dead body of any man shall be unclean"
Contact with a dead body makes a person impure.