Need help please

Stock monitoring can be done in MVD. There are many way to achieve it.

You can elaborate more on what you are working on so that someone might help you.

I want to create a form to add orders and a form that contains a table that contains the quantity of goods in the store, the purchase price and the selling price. When adding an order to the customer, the quantity of the goods in stock, the purchase price and the selling price, and when saving the order, the quantity purchased by the customer automatically decreases from the store.  I apologize for the poor expression because I am not good at English

Again, there are many ways to do it.

For the purchase price and selling price, in my experience, saving this details will only give you estimated markup(when the price differ on your next stock or purchase transaction of the goods/item and the store has remaining stock of it). You don't want your application to check the quantity of each stock transaction you made and tag each sales/pullout of the item to where stock transaction you get it from.

If you are trying to compute exact income of the store, it is better to just have a cashflow module that will handle it.
Saving all expenses and sales/collections.

Then,
Income = Total(Sales/Collections) - Total(Expenses, this includes the purchase of the goods/stock basically).

Just charge your client on the additional module(cashflow). 3:)