Check stock from external database / cashier system

[ESFHNteam]

Joomla! 2.5.6
VM2 2.0.8e


Hello,

We're working with an external database(the cashier) to get the products and stock in the site. There is build a connection with it and its working now. (thank also for OpenGlobel )

Now, we need to check the stock external when somebody clicks on the "order" button. It must be not visible for the user. We got the script for it, but don't know where to place it. The function is, that the stock will be checked on the external database and get back. If the product is there it can be ordered.

Any idea where we can place the code we have, to check stock when somebody clicks on order button?

Thanks in advance for your help.