change withholding tax base amount in S4HC

[NDG]

Hi,
I posted an invoice with wrong withholding tax base amount.
Now I need to change the value before proceed with payment.
The withholding tax is applied to the payment, not to the invoice, and I cannot reverse the invoice.
I tried to change the value from "Manage journale entries" app but the field is grayed out.
I tried to change the value in app "manage automatic payment" but it is not possible.
How can I do?

Thanks

[NDG]