What are the impacts of Refunds on the related Opportunity record?
When a refund is made we only update the original transactions Opportunity record's Total Paid field. If the Refund is for full amount then the Total Paid field will be $0 after the refund.
We DO NOT update the Opportunity->Stage field.
Also, Payments2us will not create a new opportunity for Refund Transactions as part of the standard feature. If you wish to have opportunity created then you can create a flow to create a new opportunity when a Refund Transaction is created.