All blockchain payments are irreversible, so a payment with an incorrect amount (for example, that is less or more than the amount specified in the payment invoice) cannot be canceled.  

So, if the buyer paid less than the amount specified in the receipt: you can re-invoice with a request to pay the missing amount (for this purpose, you can use the same payment address to which he paid earlier, or generate a new one). And if the buyer overpaid (paid more than the amount shown on the check): You can send the overpaid amount to the address he paid from. 

*You can find the Buyer's address on any Bitcoin transaction tracker, such as www.blockchain.com/explorer by entering the payment address you received from GCP to bill the customer*.