Updating payment instrument field for an AP Voucher

Hi,

I'm trying to update a payment instrument for a voucher that has been posted already but what happens is, the application doesn't keep the update even after it's saved.

So, from the additional info screen on the voucher detail accessed through form, I update the payment instrument field and click save. then If I go out of the voucher and re-enter it again, I found it's reverted to blank.

Is there any explanation why this is happening ?

Thanks.

Fady