There are many small details of negotiations that could be worked out. Options and incentives are obviously the biggest, but trade bonuses are also something, and even signing bonuses and salary advances are negotiable points.
If Brown intends to sign and the Celtics don’t intend to trade him, and both sides know both things to be true, there’s no actual hurry for a deal to get done. The official deadline is in October, they’d certainly like one before training camp, and ideally while everyone is still in Vegas. But sometimes negotiating over even small points requires deadlines to make people give up a position.
I’m personally in such a situation now at work. Been negotiating with a company since October. We had a soft deadline of March 15th. We blew through that and set a “hard deadline” of June 15th. But since all the time we all knew that the for real deadline was July 15th, we barely talked for April and May, and really started talking again in June 10th. And now it’s July 10th, and we’ll probably agree to something in the next few days that looks 95% like a proposal that was made in mid-January, but there was no deadline so both sides kept negotiating until they were convinced they got the best they could.
In other words, it could be about something as seemingly small as when his salary is paid throughout the year. It could be much larger also, but there’s very little that can be read into the situation from the outside.