How can I turn a $100 Apple Gift Card into cash?

Check your Apple Account balance - Apple Support

You can’t redeem or return Apple Gift Cards, App Store Gift Cards, or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support to request a refund. When you contact Apple, include the following information:

• The primary email address that you use to sign in to your Apple Account
• The refund amount
• Your current shipping address

click here ➜ contact Apple Support

For most regions, that is not possible and falls under the regulations of the Government to curb money laundering. That is true for all Gift Cards, not just those that are Apple branded.