Formula to Subtract from one column first, then another if remainder - Production Case

Hi everyone!
I was wondering if someone could help me build a formula that I am stuck trying to figure out for a few days now

I am building a business case model that shows the following per month:
- total production capacity per month
- total sale per month

At certain point during the year sale exceeds production and, therefore, production of this "extra" need has to be shifted in previous months when the sale is below the maximum production.
Below is an example:

Jan Feb Mar Apr May Jun
Maximum Production Capacity 100 100 100 100 100 100
Sale 40 65 85 110 120 90
Difference 60 35 15 -10 -20 10


What I need to do now is shift first the "extra" 20 sale in May to produce partially in Mar (because I have 15 extra capacity) and additional 5 to Feb and additionally add "extra" 10 from Apr to produce in Feb.


Can anyone help me out with setting up a formula?

Thanks in advance!
A

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