Just stumbled across the answer: According to Disney canon, the rebellion paid Han the whole sum.
“Yeah,” Han said. “The captain. A real pleasure to meet you and all, but I’m going to skip ahead in this story to the finale. I was hired by Luke and this General Kenobi to shuttle them to Alderaan and promised a reward for bringing everyone here. I have no interest in your revolution, just in getting what I’m owed. I believe the number was seventeen thousand credits.”
Commander Willard’s skin went a chalky white.
“Han!” Luke could have throttled him for being so rude. He really couldn’t wait a few hours to bring up money?
“We don’t keep that sort of money lying around,” Commander Willard said. “And this is...we need every cent to keep up our operations.”
“I’m willing to accept the amount in precious metals if you don’t have the credits on hand,” Han said, crossing his arms over his chest. “Come on, pal, you don’t want word to get around that the Rebellion can’t honor their debts, do you? No one would be willing to do business with you.”
“We do, but...” Commander Willard looked at Leia again, clearly confused. He’d been expecting an ally, if Luke had to guess, another new recruit. Luke was surprised at how bitter his own disappointment tasted; some part of him had hoped that Han would change his mind when he saw what the Rebels were up against.
But there was still time to try to convince him.
Luke couldn’t understand a word of what Chewbacca said to Han next, but the past day had taught him to read the Wookiee’s moods by his tone and expression. And Luke thought he might actually have an ally in Han’s copilot.
“Give him whatever he wants,” Leia said stiffly. The look she shot Han could have incinerated half the Death Star. “The sooner he gets it, the sooner he’s gone.”
That was exactly what Luke was afraid of.
- Star Wars - New Hope: The Princess, the Scoundrel, and the Farm Boy
Another Disney canon novel confirms that Han and Chewie were paid, but not who paid them:
Between the reward money for rescuing the princess from the Death Star and the fee they’d been promised for the Alderaan run besides, they had more than enough money to square things with Jabba. Enough, even, to get back into his good graces and have him call off the bounty hunters he had already set on their trail. But that would work only if they brought the money to Jabba; if the bounty hunters brought them in first, it would be a different situation entirely.
- Star Wars: Smuggler's Run: A Han Solo and Chewbacca Adventure
Update:
Some have expressed doubt that Han would be selfish enough to demand payment for his services to the Rebellion:
Han isn't such a dirtbag as to insist on being paid the money Kenobi promised after he died. If he had said "hey, the old man owed me 17,000 - who's going pay me back?" Luke and Leia would've punched him in the face.
- Red Caio, comment under Phantom42's answer
In fact, it seems clear that this was his modus operandi in the early stages of his affiliation with the Rebels (much to Chewbacca's chagrin). When the Rebellion is evacuating its base on Yavin after the destruction of the first Death Star, Leia asks Han to rescue a Rebel scout from an Outer Rim world. Han refuses until Chewbacca weighs in on Leia's side; even then, Han insists on being paid:
“I’m not part of this!” Han Solo said. “I’m not a part of your rebellion, I’m not a freedom fighter, and I don’t work for you, Your Highness!”...
“Let me ask you something,” Leia said to the Wookiee. She jerked a thumb in Solo’s direction. “Is there actually a heart beating in there, or just a safe where he keeps his credits?”
Chewbacca snorted, then looked to Solo, canting his head. He barked.
“Oh, no, no,” Solo said. “You haven’t heard what it is she wants us to do, Chewie. Go ahead, Your Shining Royalness, tell him about this little suicide mission you’ve got up your sleeve.”...
[Leia explains that the Rebel scout responsible for choosing the sites of future Rebel bases is being pursued by the Imperial Security Bureau and is stranded on a remote planet, with Imperial forces closing in]
Chewie huffed softly, under his voice. He and Solo both could see this coming.
“The Falcon is the only ship fast enough to reach Cyrkon in time.” She pressed the controls on the map once more and the images winked out. She turned to look at them— first Chewie, then Han. “If the ISB captures Ematt, they’ll get everything. They’ll torture him. They’ll drug him. They’ll get everything. It will be the end of the Rebellion.”...
Chewbacca growled, a short string of barks that ended in a heavy rumble.
Solo looked at him in amazement. “Think this one through, Chewie.”
The Wookiee snuffed.
Solo shook his head. “You’re supposed to back me up, not side with her!”
The Wookiee snuffed again.
Solo couldn’t believe this. “She’s asking us to fly into a system on the edge of Hutt Space to rescue a guy who maybe is already dead, never mind that the ISB is after him! Never mind that Cyrkon is teeming with the worst scum this galaxy has to offer. Never mind that Jabba’s got bounty hunters taking numbers to come after us, if he hasn’t sent them already—”
Chewbacca grunted and barked.
“I know it’s the Outer Rim! I know it’s on the way, but even if we succeed we’ll have to take him to the rendezvous point, or else it’s not much of a rescue! This isn’t our fight, pal!”
This time the Wookiee remained silent, just looking at Solo with those blue eyes.
Leia was looking at him, too.
Solo sighed. Some fights, he thought, you just can’t win.
“We’re going to need the pass phrase, whatever it is, so Ematt will recognize us,” Solo said. He tried not to sound petulant.
Leia smiled as if she’d known all along he’d say yes. Solo scowled.
“And I expect to be paid for this,” he added.
- Star Wars: Smuggler's Run: A Han Solo and Chewbacca Adventure
It appears that Han's eventual decision to join the Rebellion outright was largely influenced by Chewbacca's encouragement; the Wookiee would have joined the Rebel Alliance much earlier, if it hadn't been for Han and the Millennium Falcon:
The Wookiee didn’t fancy their [i.e., the Rebels'] chances.
That said, he understood their fire. He was a Wookiee, after all, and he knew passion. His were a proud people, a people who had lived for hundreds of years peacefully on their wooded homeworld of Kashyyyk until the Clone Wars. He had been younger then, just one hundred and eighty, and he had fought the Separatist battle droids. He had witnessed the betrayal of the clones and the beginning of the Empire. He had seen his people, his brothers and sisters, his family, put in chains and sold as slaves throughout the galaxy. He had been put in chains himself, and just the memory of it made a growl rise in his throat.
So he understood the Rebellion. In truth, he would be standing with them if it weren’t for two things: the Corellian and the ship. He wouldn’t abandon either of them. He was bound to both, as they were to him.
- Star Wars: Smuggler's Run: A Han Solo and Chewbacca Adventure
