(Inspired by this question)
On Coruscant, there existed at least one "Clone Bar", know as "79's".
This bar was privately owned, so the clones presumably had to buy their own food and drink.
This implies that the clones had at least some credits, somehow.
The clones were arguably slaves, but were they ever paid or rewarded in any way?
Legends answer: No, clone troopers of the Grand Army of the Republic are not paid
There are few mentions of reinbursement or plans for the clone troopers after the war in the book Star Wars: Republic Commando: True Colors
"So tell me what the robbery was all about." Jusik seemed to want to change the subject. "It's not like either of you to put your men at risk for personal gain."..."I reclaimed something that was due to me, but the bulk of the haul is for our men when the leave the army. You might have noticed the Republic hasn't made pension for them."
"It hasn't made provision for them to retire, either." Jusik said.
Star Wars: Republic Commando: True Colors page 129, 20 BBY
This matches well with the general attitude towards clones at the time of the war as stated by Senator Den Skeenah of Chandrila when adressing the senate,
"We have laws on how we treat sentient species. We have laws on how we treat animals and semi-sentient species. We even have laws protecting plants. But we have absolutely no laws whatsoever governing the welfare of clone troops-- human beings. They have no legal staus, no rights, no freedoms and no representation. Every one of you here who accepted this army without mumur should hang your head in shame. If that's the depths we as a Republic can sink to in name of democracy, it hardly surprises me that the CIS wants to break away. The end can never justify means like this."
--Senator Den Skeenah of Chandrila, addressing the Senate eighteen months after the Battle fo Geonosis, after setting up a charitable appeal to fund the only veterans' welfare facility in the Republic
Star Wars: Republic Commando: True Colors page 134, 20 BBY
So from this it is very clear that clones are not considered to be people by the laws of the Republic. They do not have to be paid and no future is planned for them beyond the war.
Chancellor Palpatine has this response:
"Soldiers of the Grand Army, in honor of your courage and service against oppression, you shall want for nothing, and become instructors of the next generation of young men to defend hte Republic"
--Chancellor Palpatine, in a message to all ARC troopers, commanders, and GAR commado units on Republic Day
Star Wars: Republic Commando: True Colors page 179, 20 BBY
Note, however, this is just for ARC troopers, Commanders, and Commados, not for the millions of regular clone troopers. It is never specified what he means by "you shall want for nothing", but IMO this does not mean payment or freedom.
One of the Officers in charge of training clone Commando's and ARC troopers also goes to great lengths to ensure that his men, and whoever else he can save, will have a life free of the accelerated againg process and free of the GAR. He even goes to such lengths as to mutiny from the GAR with whoever would follow him to free his clone sons.
Keep in mind, also, the discovery of this army was very suden and pre-paid, with the price being for the purchase of the soldiers, as slaves. The senate did not discuss how they were to pay salary or pension for the soldiers, they just accepted the initial price and looked the other way.
Also, the Republic is not banking on these soldiers surviving very long. They age twice as fast as a normal human being, so a clones life span (if they are Kaminoan Clones) is only about 35-50 years, and that's if they are not killed in the extremelly dangerous enviroments they are forced into. All clone troopers around the time of the First Battle of Geonosis (22 BBY) are about 10 or 11 years old, but have already aged to the point where they appear as adults due to the accelerated aging process.
As for why there were clone troopers at that bar, we don't know. At one point during 21 BBY a particular battalion of troopers is granted a few weeks as leave to cover for a covert operation on coruscant (it would have blown the commando's cover if they were seen on coruscant on leave while no other troops were) and were given some credits to buy drinks and food. This took a great deal of persuasion to convince the higher ups to give even this one battalion leave, as the higher ups wanted as many troops on the front as they could muster., so it seems unlikely this happened often.
So you are right, clones are treated as slaves.
Probably not during the Clone Wars. Remember, clone troopers are weapons purchased by the Galactic Republic. They are in effect slaves. At most, they are paid a nominal sum for living expenses, R&R to keep morale up, or basically to interact with the real world while on duty, but they're unlikely to be paid anymore than they routinely need, unlike career soldiers and officers.
After the Clone Wars though, we don't have any evidence but I suspect compensation is indeed given upon retirement (a possible but unproven example is described at the bottom). When you are phasing out millions of veteran soldiers and replacing them with raw recruits all at once, you wouldn't want than to take up arms in reaction to your retrenchment exercise...
From here onwards is purely my own guess, with little evidence to support except logic. Clones who volunteer to continue service after most of them are retired may be rolled in to the Imperial Stormtrooper remuneration scheme (since that is what they are now).
As for evidence of compensation upon discharge (along the same principles as what happened to Poe Dameron's mum when she completed her service to the Rebellion/Republic), I think Rex may have received that old AT-TE as part of his compensation. I've been asking questions about how he got hold of it while leaving the Imperial military on non-hostile terms. This seems one of the few plausible scenarios. Again, proof of this AT-TE's association to any payment Rex may have received for his service does not exist.
