Hi Sinthu Dinesh,
I'm facing exactly same issue.
I can change the FI document's Withholding Tax Base Amount using FB02 and able to change these documents manually, irrespective of open invoice or cleared invoice.
For the same case, while doing LSMW recording, I am unable to change as the Withholding Tax Amount screen is disabled.
May I know how did you fixed the issue?
Thanks,