When you set the payment rule to Cash and push the payment button, you can enter a payment amount, but, that amount is ignored and the cash payment is always set to the full GrandTotal.
found same issue with check - additionally there is an error "Current record was changed by another user, please ReQuery" if the user didn't save before pushing the pay button
Implemented also sysconfig ENABLE_PAYMENTBOX_BUTTON. If set to N in a tenant (or system) it will disable the payment button.