It's definitely not supported to update the "extendedamount" field directly, and I don't think it is possible, since CRM 2011. Your better options would be to update the fields that go into the extendedamount calculation, or to use custom fields and write
your own logic to calcualte them , and to sum values up to the opportunity
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk