It all comes down to bargaining power. Exploitation is a post-hoc label attached to a transaction where one party had more bargaining power than the other.

All transactions are logically neutral. Each party gives one thing and gets another. One could think of a commercial enterprise as a bunch of working stiffs hiring an entrepreneur to organize their efforts. That's where commercial value comes from - the organization of the labor to produce something that users value.

As it happens, there are so many potential workers and so few qualified entrepreneurs, that, in practice, the entrepreneur recruits and selects the workers and pays them as little as they will accept. But, from high enough altitude, we can't distinguish that transaction from the entrepreneur being paid as little as the entrepreneur will accept. The entrepreneur's major concession is taking the last dollar rather than the first. Still, the entrepreneur usually has more bargaining power than the workers.

Unions create bargaining power, which is why unionized workers do better than non-unionized workers. But competition can make it impossible for one employer to grant concessions to a union and still compete on price. Trade and industrial unions help address that problem, but an even better solution is political action, whereby workers "bargain" in Congress for uniform rules that don't disadvantage employers vis-à-vis each other. Political clout is, therefore, another form of bargaining power. (Politics is just economics continued by other means.)

Defining "value" is a red herring. What matters is how things get made and distributed, and justice in a free society is achieved by giving people a level of bargaining power that, by reason of the distribution that results, is sustainable with a minimum of coercion. There are lots of ways to do this. I prefer a UBI as the least invasive.