Wow!, very impressive did not know there was provision for this. Anyway yesterday while I was checking I thought it was a better idea to implement the global discount directly into the subtotal price worked out in account invoice line. So if you apply a global discount of 5% and the subtotal price of the line is 100USD, the extended account invoice line will have a subtotal price of 95USD. By doing that you only modify account_invoice, account_invoice_line and account_invoice_tax and keep the accounting module untouched (which having seen the code is something tricky to modify). (If you want to give an opinion I will listen anyway, I guess there are several ways of doing it). Anyway it is great to know that there is a hook method you can modify. Thanks a lot I might still need it.