if you are PR or citizen, CPF is compulsary. No room for negotiation
You will get salary after deducting your potion of CPF contribution (5%-13% depends)
if you are holding EP/WP, you may opt for CPF under some Bank deposit programme. Sth like CPF open an account with DBS or UOB for you. Every year you deposit some cash into that account and your employer will match that amount under CPF regulations. sth like that. I did read a bit for my employment arrangement which has this option.