Full Idea
The fundamental flaw of liberal justice, Marxists claim, is that it licences the continuation of the worker by the capitalist, since it licences the buying and selling labour.
Gist of Idea
Marxists say liberalism is unjust, because it allows exploitation in the sale of labour
Source
Will Kymlicka (Contemporary Political Philosophy (1st edn) [1990], 5.2.a)
Book Reference
Kymlicka,Will: 'Contemporary Political Philosophy (1st edn)' [OUP 1992], p.171
A Reaction
I can't see that all sale of labour is exploitation, if (for example) the wage paid was extremely high (maybe even higher than the employer's wage, which is possible). So exploitation involves something more.