Update Quantity in record of lookup

Update Quantity in record of lookup

I have a attached a test application to work out a problem I'm having.

As a row item is added to an invoice , I'm trying to subtract the quantity from the underlying lookup record quantity.

I attached the code:

tempQty  =  Products  [ID == input.ID];
tempQty.Inventory_Quantity = (tempQty.Inventory_Quantity  -  input.quantity);


To the OnUpdate area of input.quantity of the subform.

any help would be appreciated !

Thank You.

Chris