There is a setting that is ticked in Sales Ledger>>Settings>>Account Usage S/L>>Force Chronology for Invoice, that overrides the details in the Access Group that you belong to.
If you un-tick this option, you'll be able to change the date on the Sales Invoice.
Chronology forces the user to do various actions in sequence, e.g. you can cannot Receive Goods in stock prior to a date that those goods were last invoiced.
You need to know why Chronology was set on in the first place, you cannot simply tick and un-tick this setting as that will result in problems with your Average cost prices.
HansaWorld Support Team