Suppose you went down to your local animal shelter or homeless shelter and asked:
I would be happy to volunteer 10 hours every weekend. I will perform general-purpose labor, meaning I will do any task you set me on.
— OR —
I can stay home and write you a check for the equivalent of 10 hours of general-purpose labor. At a local market rate of, say, \$15 per hour, that's a donation of \$150 every week.
Which would you prefer?
We don't even need to speculate. People have actually done this, and the response is instantaneous: "Oh, we would prefer the cash, thank you."
Don't economists have some law or theorem that says the value of labor and the value of capital should equalize, and thus the shelter should have no preference about whether they receive a donation of labor or cash?
Or is it that in this particular bounded microcosm, there is a preference for cash because they can use the cash to not only purchase labor if they need it, but also things like supplies, rent, etc.? Does this preference go some way to explaining Piketty's puzzle about why r > g? Capital is preferred, and therefore rewarded, over labor.
Is there some other bounded microcosm in which they would actually prefer the labor? Maybe some labor-intensive industry, like picking strawberries, harvesting sugarcane, or hand-stitching soccer balls together.