PCR says
" Summoning: A summoning spell instantly brings a creature or object to a place you designate. When the spell ends or is dispelled, a summoned creature is instantly sent back to where it came from, but a summoned object is not sent back unless the spell description specifically indicates this. A summoned creature also goes away if it is killed or if its hit points drop to 0 or lower, but it is not really dead. It takes 24 hours for the creature to reform, during which time it can't be summoned again."
So, if you summoned the cat, and you could get it to not run away, you could shave it and then cast cat's grace before the summon vanished, in theory. But I seem to remember a conversation like this where they mentioned summoned monster parts couldn't be used as spell ingredients. And since in the case of slaying, the body instantly vanishes when it reaches zero HP, and presumably doesn't leave any parts laying around, that wouldn't work either.
