I suspect I’ve been scammed. How can I dispute transactions?

If you sent Apple Cash using Messages to a person, the only means of getting your money back is by asking them to send it back to you. It is a private transaction between 2 parties and just like giving someone on the street cash. That is why Apple says to only send Apple Cash to people you know.

Apple Cash person-to-person payments are like any other private transaction between two people, so only send and receive money with people you know.

Send and receive money with Apple Cash - Apple Support

For a scam ad that Facebook allows on their platform, report the ad to them here:

https://www.facebook.com/business/help/162606073801742

Reporting fraudulent users or content can be done here:

https://www.facebook.com/help/263149623790594/

Presuming that you sent Apple Cash to someone you do not know,

if the cash has not been accepted, long press on the amount in iMessage for a menu from which you can cancel the payment.

if the cash was accepted, you can only ask for it back.

No legitimate business will accept Apple Cash direct transfers. It’s against both merchant terms and conditions and also individual terms and conditions.

“The Services may only be used for personal, non-commercial purposes.”

https://applecash.greendot.com/termsconditions/

It probably was a fraud. Lot’s of people like scamming people. You can access contact someone and maybe that person could get it back.