Opening and Closing Inventory Balance - Forecasting

Hi all,  new to the forum.

Im having trouble dealing with closing balance of inventory when the forward forecast results in negatives.

I have:

-current starting Inventory

-monthly sales forecast

-monthly incoming inventory

Im using the following

InvMovement:=[PURCHASES]-[SALES]

OPENING INV:=calculate([InvMovement],FILTER(ALL('DATE'[DATE]),'DATE'[DATE]< MAX('DATE'[DATE])))+[CURRINV]

ENDING INV:=calculate([InvMovement],FILTER(ALL('DATE'[DATE]),'DATE'[DATE]<= MAX('DATE'[DATE])))+[CURRINV]

the problem is that there cases where my Sales Team is forecasting above the available inventory for a future period.  When we are out of Stock we still want the forecast to remain, even though we cant fill the sale.

The above OPENING INV and ENDING INV calculations work when inventory is always positive, but accumulates the negatives on Out of Stocks and results in incorrect (negative) opening inventory positions.  

Id like the ENDING INV results to be 0 if it calculates as negative and ENDING INV of the following period to also be zero.

any suggestions?

Thanks