Subtract value if value matches that of another cell

Hello all, and thanks ahead of time for any help you are able to provide. New to what I would consider more complicated Excel formulas, so I'll try not to confuse with my request!

In the attachment, I'd like to populate column N on the September 2022 sheet with something like =M2-'August 2022'!M2 but only if the value in column B matches. The problem is, I need to do this month over month and the values in column B will not always abe in the same order. I believe it will need to match via VLOOKUP?? I hope this makes sense, please ask clarifying questions as needed.

Thank you!