In the Marxist concept, animals, machines and slaves are incapable of producing surplus value. They only produce as much as they have to in order to function, and no more. Doesn't this also describe the condition of "wage-slaves"? I have heard that the difference is that the wage-slave sells his labor. But I fail to see how a machine that sells its labor for fuel and spare parts would suddenly be able to produce surplus value. Explain please.
This question is oddly phrased.
The main marxist thesis about profit is that firms can't profit from technology, but ultimately profit comes from exploitation of human labor, i.e. by extracting surplus value from workers.
In marxism, profit always comes from "living labor", in the sense that the price of a commodity both reflects the technology used to create it and also the human labor congealed in it. However, a firm can't "pay less" machines because machines have fixed values while a living human being can be paid less than their value of their labor if the value of their labor exceeds the value the worker needs for subsistence (e.g. sufficient for their bills, calories, leisure etc).