Remaining leave calculation in M

Dear all,

I have now been assigned another task that gives me sleepless nights. And I hope you can help. (For compliance reasons I am unfortunately not allowed to share the whole excel file with all employees information but I have inserted a draft of what I have and what I am missing.)

So, let's get to the jigsaw:
My boss asks me to built a query from a various sources (sickleave list, employees' contract terms, company holiday). The goal is to automatically have calculated the remaining leave of our employees. I have come a long way already but I am missing the last bit. If you take a look at the excel file, it boils down to a sumif-complexity-issue (I think at least). The green figures in the sheet I have already, but the red ones give me a hard time… My problem is that my draft looks nice, but try to delete the months after the current month: Then everything goes south and the remaining leave for the current month does -of course- not work anymore-

Anyone, an idea on how to solve this puzzle?

Best,
Christiane