Is there a sin or expiation upon one who forgot a chicken in the trunk of his car, then lost it?

There is no sin or expiation upon you if you placed the chicken in a spot where you thought it would not fall, for Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended." However, if you placed it in a spot from which it was likely to fall, then you are mistaken and negligent, because that leads to the prohibited waste of money. In that case, the expiation is repentance.