We have found that when bringing over a PO Receipt into a Purchase Invoice that the user can change the quantity and invoice for more than what the receiver brought into inventory. 

The user in our system is presented with a warning:

"Warning - Maximum quantity to be invoiced XX EA" -> where XX is the quantity from the PO receipt

I did find this entry which talks about restricting the receipt to only allow up to the amount on the PO but I could not find anything for purchase invoices:

https://sagecity.na.sage.com/support_communities/sage_erp_x3/b/tips-tricks-by-greytrix/posts/how-to-restrict-user-from-creating-receipt-greater-than-the-po-quantity

Thanks,

Mike Rohwer