Welcome to the PayPal Community and thank you for your post!
I do understand your concern with this. When you make a transaction using PayPal, you won't have the option to cancel it yourself. This must be done by the merchant.
If the transaction shows as Completed in your PayPal Activity page, this would mean the merchant has received your payment and not canceled your order. In this case, I suggest you contact the merchant directly to request a refund. As they have received your payment, they will have the option to refund you.
Once the refund is processed, you will no longer owe the amount on your PayPal Credit account.
I hope this helps!