You can create multiple refunds for a transaction. If it was paid by Credit Card then the total sum of refunds can not exceed amount of the transaction. For bank-link paid transactions there is no such limit.
See also the State model of refunds
Note that you should verify state of the refund in the response. You may be returned 200 response (resource created) but it's state may be
While the refund is in state
it's amount is reserved in shop's account.
You need sufficent balance on the account to make refunds.