Unless I'm doing something terribly wrong, it appears Marshal Forwyn's interactions are broken.
I was offered to upgrade my army for the assault on Lord Gathbin's army, but was unable to afford it.
I now have the money available so return to him to pay but after selecting the same option that activated it previously: 'What's the status of my regular troops?', the option is no longer there to upgrade via the 10'000 gold.
Should it be useful, I've uploaded a save file to the following link: http://www.filedropper.com/231a81638a8248c19b47e8b8261bdb9113991955greathall