De Gea is getting old and maybe that's why Manchester United tried to decrease his salary for the next season since is a famous payer and achieved many goals for this team. It was not good to ask this from De Gea however I think it was not about salary but that's mostly because of Ten Hag, he prefers to work with the goalkeeper he had at Ajax
Not only that De Gea is getting old, De Gea is at the same time doesn't fit in, with Ten Hag's modern kind of goalkeeper. That was among the reasons Manchester United moved for his salary to be decreased. Instead of accepting the pay cut, he prefers to be a free agent instead of accepting the new offer given to him.
To Manchester United, they felt like, it is preferable for them to sign another capable goalkeeper named Onana from Ajax than to renew De Gea's contract with the same high weekly payment, of which he has lost focus and doesn't fit in today's modern goalkeeper who can hold and pace ball without fear