[Pamar]

"Coercitive" would be something like "you make N license plate a day or don't get food".

"If you accept to do non-mandatory work in exchange of money/reduced sentence" it will be your choice.

[maxmamis]

another way to rephrase that is "if you choose not to work, you will stay in prison for longer." sounds pretty coercive to me!

[Pamar]

Not longer that your original sentence though.