Inventory Forecast Recursive Calculation

Hi all, I'm trying to create a DAX measure that populates "forecasted remanining inventory".

I have gone through Gerhard's blog on recursive calculation, but it talks about compound interest and not summation.

In Gerhard's blog, he replied in one of the comments saying it "should be easy" but I haven't been able to figure it out for 3 days already.

I have gone through this thread but I don't understand what the user wrote, nor the answer derived.

My attempt is:

Forecast Inventory = 
VAR PrevInv = CALCULATE ( SUMX(Table, Table[Inventory]), DATEADD( DateTable[Dates], -1, MONTH))

RETURN 
     IF ( ISBLANK ( Table[Inventory] ) ,
           PrevInv - SUMX(Table, Table [Sales]), 
           SUMX(Table, Table [ Inventory] ) 
       

I know that the "PrevInv" variable is not calculated correctly but I really don't know how else to proceed. Can anyone please help?

Assume that a DateTable has been created separately and is configured properly.

Thank you!