Decrease product amount when promotion is added with larger amount then subtotal

Hi,

We ran into a problem.

We have a site where you can use your wallet value to buy some products. The Wallet value the customer wants to use do we call a Wallet Promotion.
The customer selects the Wallet amount (max amount of order subtotal ex vat or max of customer wallet amount) en after that we create a Promotion of type WalletPromoData (override from OrderPromotion). 

Now we have the following scenario:

1. Add product to cart with 4 seats a E200,- each (order subtotal is E800,-)
2. Use Wallet value for whole order, so also E800,- (promotion with couponcode is generated and visible)
3. Decrease the seats to 2, so order subtotal will be E400,- while the Wallet Promotion still has a value of E800,-
4. Now, the cart page run into the following error after calling OrderRepository.Service.Save(cart): "Invalid order group total calculation"
5. So it looks like Episerver doesn't decrease the amount of the promotion to the max of the order subtotal

Does anyone else ran into the same problem or something like this?

Thanks in advance.

Grtz Sander