The invoice must be amended September 2016-December 2016 = 4 months January 2017, at no cost, think 1 month = February-March 2017, 6 month I understood correctly?
Invoice must be resolved as of September 2016 - December 2016 = 4 months from January 2017 at no cost idea = 2 months from February to March 2017 after six months my understanding is correct.
Invoice must be solved in September 2016 - December 2016 = 4 months January 2017 at no cost, think 2 month = February to March 2017 fully 6 months. I understand is correct or not.