Keep track of movements, not of inventory. I.e. you have all incoming quantities and amounts as positive values and all outgoing quantities and amounts as negative values.

To calculate current quantity of article in stock, you sum all quantities for this article - same for monetary amount for article in stock.

To calculate the quantity (and amount) of article at given date, you sum all quantites/amounts for this article with movement date <= this given date.

To calculate the balance for given period, you calculate quantity/amount at end of period and deduce the quantity/amount before start of period.