If the facts you state can be established, you have a contract for the purchase of the laptop, and he cannot simply ignore its terms and force you to return it as long as you do not breach the terms of the contract. That means making your payments on time.
Keep in mind that verbal contracts are fully enforceable. The difficulty with verbal contracts is proving their terms. If your former friend files a criminal report, you need only show the police your documentation and tell them about your deal, and they will not prosecute.
Good luck to you.
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Here is a sample response (based solely on the facts your post):
Sorry we had a falling out. Perhaps one day we will get past it.
I intend to continue to make payments to you as promised.
Good luck with it.