Can Employee/Employer or Landlord/Tenant or Lender/Borrower relationships become manipulative?
Yes, just like any relationship can. Depending on the society they're established in, they may even
start that way, with one party having a clear advantage, such as a high unemployment rate giving employers an advantage, while a severe labor shortage gives the advantage to the workers.
Does that mean that they are
inherently so? No. No mutually consensual relationship is inherently manipulative.
While we're at it, Let's look at the dictionary for a sec...
noun
1. use or utilization, especially for profit: the exploitation of newly discovered oil fields.
2. selfish utilization: He got ahead through the exploitation of his friends.
3. the combined, often varied, use of public-relations and advertising techniques to promote a person, movie, product, etc.
Now, It's obvious, that when McGruder and other Proudhoun anarchists use the word, they mean the second definition, "selfish utilization". And on the face of it, they're entirely correct. An employer selfishly utilizes his employees for profit. That's what the business is there for. But they conveniently ignore one crucial fact, that I pointed out, before: The employee selfishly utilizes the employer for profit, too. He turns hours of his day, that he otherwise would have spent doing something else, into money, which he otherwise would not have gotten, or at least, had to get in another manner.
This also makes one very flawed assumption: that being selfish is somehow bad. It's not. That's not to say altruism is bad, because it isn't, it helps the survival of the society, as well as the species as a whole. One cannot be selfless all the time, however. At least some self-interest is required for the continued survival of the individual. Obviously, if all the individuals die, the society is dead too, and along with it, the species. So each person must look after themselves first, and others second.
Combining this concept with the knowledge that things go a whole lot easier if everyone cooperates, and you get the free market: Everyone puts out there what they want, and what they're willing to offer. When someone finds someone else who happens to be offering what they want, they enter into negotiations to get it, haggling until both people agree that the deal is acceptable. Since both people are better off than before, both have profited.
Both parties benefit, but one much more than the other, because one has more capitalist power than the other. If employees, tenants, and borrowers had an even playing field, they would own their workplaces, their homes, and wouldn't pay interest. But they do not, because capitalism reigns.
Could you please explain, exactly, how an employer gains more from his employee than his employee gains from him? (we'll stick to one example case, for clarity)
Employee spends eight hours building a widget. Employer pays employee 80 bitcoins (10 BTC/hour) and takes the widget. Employer sells the widget for 800 bitcoins. The employer hasn't added any work to the widget. It's the same as when he got it from the employee. The employer gets 720 bitcoins without having done any work. The employee concedes those 720 bitcoins and submits to his employer's direction because the employer will prevent him from working otherwise. Even if his former employer doesn't blacklist him, the worker will encounter the same deal with every other employer.