Hmmm that's an interesting topic, thanks for bringing this upz it's something I never thought about.

So without thinking this through just eyeballing it here's my take;

Prisons are expensive and the inmate needs to still make up for his crimes. Also being imprisoned without any kind of productive activity could have negative impact on mind and body.

Now there need to be sone rules:

• Additional qualification or study that are beneficial for the resocialisation and could lead to higher future income or better chance of getting a job should have priority.
• Pay should be fair and reasonable, the pay should be split in a fair way is the inmate needs to pay for his crimes (restitution etc). There must be a minimum remainder for the inmate so they have money available while imprisoned and the rest should be made available when they are released.
• Working conditions, breaks, OSHA etc needs to be on par with the standards outside of prison.

Now is enforcing labour still considered slavery? Could be but at least in Europe you lose some (not all) human rights when you've been sentenced to prison so it's acceptable.

Now one disclaimer: This is with only theoretical knowledge about prisons on western Europe. The US prison system is something else all together.