He's probably hoping to increase his value with a good World Cup. No need to make a decision until the end of the month.

How does it work in the NBA? If he takes that offer and then they dump him, can he keep the entirety and add whatever he might get playing elsewhere?

In the NBL, if you have a salary and are cut, the subsequent salary partially replaces what the previous employer owed.

ESPN Insider ran the following story last night.

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Free-agent center Aron Baynes prefers to re-sign with the San Antonio Spurs. However, with just four weeks to go until NBA training camps begin, both sides are not close to a deal. With that in mind, Baynes is looking at other options.

According to Ismail Senol of NTV SPOR in Turkey, via translation from RealGM.com, Baynes and his agent are checking out potential offers in Europe and hoping to get 1.5 million Euros for the 2014-15 season, which converts to about $2 million.

Last season, while making 53 appearances and averaging 3.0 points and 2.7 rebounds per game, Baynes earned the league minimum of $788,872. The Spurs do own his Early Bird free-agent rights, so they could match his asking price if they want. Best guess is that they likely are only willing to pay the $1,115,243 qualifying offer for 2014-15. Baynes has until Oct. 1 to accept the qualifying offer, unless he and the Spurs agree to extend the deadline.

Should Baynes sign overseas, the Spurs will retain his rights since he is a restricted free agent.

San Antonio currently has 14 players who are locks to make the opening-night roster, and the Spurs have traditionally kept the 15th spot open to add a player mid-season. That philosophy may not change, should Baynes elect to play overseas.

Issac.
If they cut him after he signs the qualifying offer he gets to keep all that money and then sign a new contract elsewhere.

Hard to believe another team isn't willing to sign him for $2-3mil.
Maybe they are put off because the Spurs can match any contract.
Still 6'11" skilled bigs are usually in high demand so seems weird he hasn't had some decent offers by now.

http://www.poundingtherock.com/2014/8/25/6061677/spurs-aron-baynes-contract-sign

That's a pretty good breakdown. Another team has to give him a 2+ years guaranteed contract. Even stranger is that if he does sign the QO for the upcoming season he'll be in the same spot next season as he will still be a RFA, so he'll be in a very similar position.

Basically the only leverage he has on the Spurs is "I'll go play in Europe" which still isn't much leverage (because if that's what he really wanted to do he'd have kept doing it, instead of switching over to the NBA).

I expect holding out and waiting for the World Cup is the advice his agent is giving him. Is it possible for the spurs to withdraw the QO? I presume so. I just hope it doesn't blow up in Aron's face if so. If he goes down with an injury in the world cup, he could well have nowhere to play next season and be rehabbing on his own coin.

I would expect if Baynes's pre-WC performance is any indicator of his performance during the WC that he will be receiving a lot of interest and hopefully some very good offers from NBA teams and Euro powerhouses over the next two months. The way he has played so far as the Boomers best and most consistent scorer and rebounder should give teams confidence that he is deserving of a bigger role than he has had with the Spurs so far.