Not true. Jay was going to take the position whether they gave up their portion or not. It was Roc-A-Fella records he would be giving them instead.
Jay got offered that position because during his career, he never cashed any checks from Def Jam, as he felt the money he made ghostwriting, touring, and being 1/3 CEO of the Roc was enough to live off. When he retired years later, and came to Def Jam to finally cash in on this large sum, it would financially cripple the company, so they offered him to be CEO and President of Def Jam, full ownership of Roc-A-Fella Records, while also receiving the masters to all his other albums at the end of his tenure.