There may be another solution. Is your general threatened by constant rebellions? If that is the case, you might want to get an emissary up there. That way, if your general retreats to the fort/keep/whatever, your emissary can bribe the rebels. I don't know if that would be cheaper than a ransom or not, but at least you get some more troops for your money, and if you can bribe enough rebels, then you will be able to hold down the province, especially if you can get a spy up there too. If he can hold the province, then your general can wait until your navy can get him home, or maybe he can even fight his way home, right through England, and across the Wessex-Flanders land bridge.