Hi Peio,

This should get you part-way there; credativ have recently produced a module called account_invoice_date_paid which records the date and time at which invoices become paid in a new field called 'Date Paid', and displays this next to the 'Due Date' in the invoice form view. At present, it does not include a column in the PDF report, but it provides the groundwork for this and the field can be included in a CSV export. The module can be obtained from here: https://github.com/credativUK/credativ-addons/tree/addons-7.0

Most PDF reports in OpenERP version 7 are written in an XML-derived language called RML. Unfortunately, there is no easy / non-technical way to customise these reports, but the ideal way to do this would be to create a new small module which depends on account_invoice_date_paid and replaces the invoice report RML file with a new one, which would be a copy of the original with the new column incorporated (the field can be referenced using "[[ (o.date_paid) ]]"). Please refer to the OpenERP documentation for further details; we apologise that we cannot provide a more comprehensive answer here, however we hope that this puts you on the right track.