Whenever a buyer says that, immediately contact Fiverr’s customer support, tell them the buyer ordered accidentally, provide a screenshot where the buyer says that and ask them to cancel the order for you without affecting your order completion rate.
They will tell you that it may or may not affect your rate but there’s at least a good chance that it won’t.
If you cancel yourself, it will affect your rate, and if you do weird things like sending someone money outside of Fiverr (why would you do that, you didn’t even get that money, Fiverr did, and you’d only get it after completing the order, which won’t happen if it gets cancelled). Also, the latter most probably will get you a warning because you violated Fiverr’s ToS, and it might even get you banned.
Never agree to contact a buyer or pay or receive money outside of Fiverr. That’s like, rule #1 here, as you should know. Please do read the ToS thoroughly, so you won’t do anything else that’s against the rules because the more warnings, the higher the chance of getting permanently banned (depending on how they evaluate a violation, they might even ban someone for the first violation), so, again, make sure to read and understand the Terms of Service.