On the Payment Txn object there is a validation rule set  "Card_and_Amount_Update_cannot_reduce" they need to make this inactive.  It might be active, which is causing this as its organisation specific  feature. Please follow the link below:

https://help.payments2us.com/m/userguide/l/1497889-how-to-enable-card-holders-to-update-their-card-details#restricting-the-donors-from-reducing-gift-amount