When our system receives notification of completed payment from the payment processor, we compare the price paid for each item in the order against those items' current prices set in Seller Admin at that moment.
If this comparison finds a mismatch, our system does not process the order, and instead sends you a Suspected Fraud notification containing the full order data, so you can decide how to handle the situation yourself. You can also look in your payment processor account history (e.g. PayPal History) to verify and investigate the payment further. Sometimes a price mismatch can happen innocently if you just so happened to change a product's price after the buyer had taken their order to checkout but before their payment was completed.
If you decide that buyer's order is legitimate, and if they had purchased a download, you can fulfill the sale and issue them a download link manually by going to Seller Admin > Manage Buyers > Send Free Download Link.