The PaymentExpress back-office application allows users to
initiate voids and refunds of card transactions. A void is a
same-day reversal that stops money from moving. A refund is an
after-same-day reversal that returns money to the cardholder.
Your system of record can use the Reversals API to initiate
voids and refunds in real-time using the paymentId of the
transaction. Any void or refund actions will be reflected in
your elected Post-Payment Operations.

Payment-level reversals always reverse the entire payment amount.
The amount field in the request body does not control how much is
reversed - it is used only for validation to ensure mutual
understanding between systems about the reversal amount. When omitted,
a full reversal is performed without validation. When provided, the
value must match the current payment balance or the request will fail.