I think you are on the right track. If I understand your problem right, this is what I'd do -
- Create an Account where the Account Type is Cash Account
- Then transfer money from your existing account into this new Cash Account
- And as you buy stuff, just enter the expense as you would, but set the Account in the expense entry as this new cash account and the total amount in the cash account goes down and so on
Let me know if this works or not.