Customer refunds through Cayan are done on a per transaction basis, not on the total contract value. For example, if you have a contract value of 100.00 and you make a payment of 60.00 and one of 40.00, if the contract is due a refund of 60.01 or more, you must do it in more than one refund as the highest amount taken in a single transaction is only 60.00.
Cayan has stated that this is for the safety of the merchant, to protect against refunding an overly large amount accidently, and then having to fight to get the money back from the customer.