Claiming over payment of $10,000 on a refund and locking computer to get the money

Someone who says that they are the owner of your business called my Father-in-law (who did business with your company) that his brother was doing something financially illegal with the company, I am unsure what. Anyhow the gist of it is that my Father-in-law received $10,000 in his account and the call stated that your company could only transfer in increments of $10,000. Therefore, my Father-in-law would need to make it right and pay back the overage. He was to purchase gift cards and call back with the numbers. Which my Father-in-law did. Then the scammer called and said he did not receive the money so my Father-in-law would have to wire him more money to cover that money not received. Today the scammer locked his computer and wanted more money. We think the scammer has gotten at least $6,000 from my Father-in-law. Who is on a fixed income and is impaired by multiple strokes. Please stop this from happening to others