Lucas tried to change the terms of the deal a few times and was met with threats by Iger to collect a fee for backing out of the sale.
If he came to Disney from day 1 with a demand, they could have sat down about that and guaranteed the sequels he wanted. Instead they sat down about the sale in general, which Lucas tried to mislead Disney on-- and they weren't having it. It's Lucas fault for entering negotiations regarding a full sale only, rather than bringing his terms to the table from day 1. Iger later placated him and agreed to use his outlines, and Lucas ultimately agreed to the sale.