You'll need to learn how to use Manage | Scripts to create scripts.
Show Custom Dialog is a script step that can be set up to display a message that includes an input field for receiving info. It can pop up this dialog, you enter your data into that input box and then click OK. the script can then duplicate the invoice and its related invoice data records (that record the line item details of your invoice in a portal) and assign the date from the custom dialog to the new Invoice's date field.
But this will not be a simple script to create for some one new to scripting.
Take a look at the example script in this thread: Duplicating Bill Of Materials (duplicating portal line items)
Just like an invoice record with a set of related Invoice Data records, this script duplicates a parent record and then also the set of related child records.
If i were to give you the file would you be able to create the two scripts i would need?
It's actually one script and the custom dialog may not really be necessary.
If you are willing to hire me as a consultant, maybe... click my folder and send me a private message and we'll discuss that privately if interested.
Sent a PM