Hi,
Can you please tell us what was the intial flow including the investment flow and the what is the value you are trying to edit to because I am not sure if the total of the final repayment flow does not match the investment, then if will through an error. And also in general, the final repayment flow is auto calculated based on the other 2 flows. Hence you will get that error.
Can you please check what is the position change category assigned for the withholding tax update type in configuration.
Regards,
Ravi