It’s been a while since I’ve used the Envelope Budget sheet as I moved to the Savings Budget sheet, but I think I understand what you are describing.
Am I right in saying that this is additional unbudgeted income that is augmenting your budgeted income? If so, this should be included on your Transactions sheet, and the unbudgeted income show in the Available column, and in future months under the category you specify on the Categories sheet under Rollover To and in the Rollover and Available columns in following months. If you have the income set to roll over to that same income category, future months won’t show that rollover as available income budget, however. In order to allocate it to a specific category you will need to use the adjustment shortcut notation to move the rollover to the spending category.
For example, let’s say in June you assigned $100 of income to the income category Extra Income, and the category is set to roll over to itself. This wasn’t expected income, so it won’t apply to your balanced income/expense budget, but you will have $100 in the Available column, and this will carry forward to any future periods as Rollover Savings. You do the same in July, so now you have $200 of available Extra Income. In August you don’t have any Extra Income transactions, but that money is still available. Let’s say you exceed your Groceries budget for the month by $150. The Monthly Budget sheet will warn you that your expenses are exceeding your budget by $150, but you will also see $200 in rollover savings. In order to balance your budget, you would move $150 of rollover from the Extra Income category to the Groceries category using r-150 in the Adjustments column under Extra Income, and r+150 under Groceries. This will balance your expenses, and you will have $50 of rollover savings remaining.