If you ask RGMers, they actually think Fox is more likely to be moved for Simmons than Haliburton. Who knows what their FO thinks, but they could be pushing Barnes/Hield, Mitc , Bagley and picks. Whether than offer is compe ive with what is actually being laid out by interested teams, we don't know.
So... If the Sixers are willing to move Harris, they are likely trying to use his salary as ballast to bring in a star or whatever. Even if the Spurs aren't providing an upgrade, they are able to provide the expiring contracts in order to prevent a third team from having to take Harris.
Like say they were going to go all-in on Jamal Murray (not actually the worst idea if they are going to keep Simmons/Embiid). Denver definitely doesn't want to come back in that deal, as most of the motivation for trading Murray would be to free up money for MPJ. So the Spurs being able to turn Harris into expiring contracts has value to both teams, even though the Spurs' offer I gave wouldn't be what Philly wanted one-on-one. Repeat the same idea with Sacramento and Hield/Barnes. There are a few trades where the other team probably wants or will be forced to take Harris on his contract like Portland (McCollum). But insofar as he's moveable, it'll likely be that they sell him or use him as ballast in a huge trade with a club that would probably prefer he not come back in the deal. SA is one of the few teams where Harris on his contract would actually make sense.