Net Amount to draw in Down payment to draw window

Hi,

My issue detailed below;

Down payment invoice : 10000

With holding tax          :  1000

Net amount                 : 9000

outgoint pymt               : 9000

When i make an AP Invoice and while trying to link down payment invoice, in down payment to draw window am getting only Rs 9000, instead of 10000. this is happening only for one vendor.

Please help to identify, where do i went wrong.....

thanks

Nobin