This is the scenario:

I have a product "hammer", which I have the following inventory (At hand: 7 units, Forecasted: 7 units). 

So a client comes in asking for 5 hammers, the salesman makes the sales order and the client goes to the cashier and gets his invoice. He thens gets delivered his 5 hammers and leaves. 

That transaction would leave my inventory as follows (At hand: 2 units, Forecasted: 2 Units).

Now Im gonna make a purchase order for 5 hammers. I confirm my purchase order and my supplier is going to send the products on 15 days.

That transaction would leave my inventory as follows (At hand: 2 units, Forecasted: 7 Units).

Then a new client walks in on a date previous to the arrival of my new batch of hammers and he tries to buy 5 hammers. When the salesman add the 5 hammers in the Sales Orders no warning shows up telling him he is trying to sell 5 hammers when he only has 2 at hand. But if I try to sell more than 7 hammers, which is my forecasted inventory, a warning does show up. 

Is this a bug, or is it easily configurable so that the "not enough product at hand" warning shows up when trying to sell more products than the ones at hand and not the forecasted one.