A buyer accidentally placed order, and now they are asking me to cancel it or send them money.
Contact Fiverr CS for this issue and Cancellation.
Will it be displayed as a cancelled order on gig?
This could potentially be a scam. Do not send money. It is possible for them to do a chargeback via PayPal at a later date - meaning they get their own money back as well as the money you send them.
If this is genuine, ask customer support to cancel the order without it affecting your profile.