change of payment conditions in written and saved document

A user need to change payment conditions in already saved document, because his customer didn't pay in time. For example it was 30 days net, but the customer payed later and that's why has to pay 10 per cent of invoice amount more. A user is not able to change saved and written document.  How could I program such an increasing of invoice amount in Accounting Suite?

Cancel Posting - Change document - save and Post. Does it work?

The system doesn't allow to cancel posting of documents containing inventory items. Currently we are working on the possibility of re-posting invoices generated from goods issue and goods receipt documents.

We implemented the ability of voiding sales and purchase invoices containing inventory items if they were created based on goods issues and receipts respectively. This functionality will be included in the 1.1.0.24 release.