Have a calculated field roll up under a different month than original record

Does anyone have a good suggestion how you can break out the output from a calculated field to either post to a new record or somehow have it roll up under a different month?

Use Case:
Budget Owners have to input [Estimated Amount] and [Expense Month], let's say it is $100 in Month 1.

Once the final invoice arrives they'll have to input that amount in the same record but in the fields [Final Amount] and [Invoice Month], let's say that is $110 in Month 2.

Since the books for Month 1 have already closed we'll have to post an adjustment of the difference in Month 2.
There are Formula fields which will populate an [Adjustment] = $10,  [Adjustment Month] = 2

From a roll-up point of view, how do I get this record to add the different amounts to two different Months?
I figured one way would be to automatically create a child record of all adjustments and have that summary field to be added.
All reporting will look at the total cost by [Expense Month], but I'm not sure how to get the [Adjustments] by [Adjustment Month] to get added to that.

Any clever suggestions are very welcome!

Thanks,
Freddy