Okay these type of things sometimes happen with new sellers (not common but rare). But coming to your situation, never accept the order cancellation request if you think the buyer is using you or manipulating your service just because you are a new seller. Simply contact the customer support and share them your experience. They will definitely cancel the order from their side, but it will not put any impact on your profile, neither your order completion rate will decrease (Note: you must have a valid point to express that you are on right side). So currently as the order is already cancelled by mutual understanding (I guess), you can still contact customer support and describe the scenario. Hope they could help you out.
Thanks and have a great day 🙂