What if a buyer said to pay money back after order completion?

What if a buyer said to pay money back after order completion?

I have done all the work but mistakenly delivered only 10% out of 100.
After delivering order I send all the work to my client on order page.
After five days, the order automatically marked as completed.
Now client is messaging me to pay 50% money back to him.
What should I have to do in this case.