How to redeem cash from Shenzhen tong metro card ?

Is it available to redeem the cash inside the metro card, value remains each around CNY 50-58.
I have two cards, since I left Shenzhen from Shekou Port line and it can't give them back to the service center. The staff told me to give them back to 3 metro stations, is that true ?

Your valuable assistance will be appreciated.

thanks and regards,
ikadewi