Marx's labour theory seems to define surplus value as being produced exclusively from human labour. One discussion of this concept stated:
Machinery, and tools of production do not create surplus value; they are a product of previous labour which store value, passing their value onto the finished commodities; they are ‘dead’ or ‘constant’ capital. Only labour (variable capital) can produce a surplus as human agency either stores the value in machinery or extracts it; in the process boosting efficiency and profit margins. source
As a simplification; If the value one assigns to a commodity is based on the human labour someone is willing to exert to obtain that commodity, then markets will tend towards an equilibrium where all commodities have relative value based on the time people are willing to invest to obtain them. The profit to be made on the production of each commodity is then derived by exploiting the workers such that the commodities they are given as wages have less value than the commodities they are creating. E.g. They are paid a wage equal to 4 hours of their work, even though they actually work for and thereby create a commodity worth 8 hours of time. The capitalist thereby gains the 4 hours of value from each employee without providing the requisite human labour. Leaving aside here arguments around trying to quantify the value of one workers hour of labour versus anothers.
However, I feel as though I don't fully understand this concept. As a thought experiment, imagine that a human workforce can produce 1000 units of value in commodities whilst only being paid 500 units of value. Here the capitalist who owns the means of production makes a 500 unit profit. Now suppose a machine was created for 100 units of value, which then produced the same 1000 units of value in commodities due to the machines ingenuity. Why isn't it possible to say that the capitalist profits 900 units of value here?
I appreciate that in the extreme of total automation of commodities there would be no employment under a capitalist system and therefore no one could buy the commodities and as such their value would fall to zero. Is this why it is said that surplus value can only come from human labour? I feel like there is more to it than this and I'm looking for an example which continues the story of this value creating machine and ultimately shows how that scenario actually creates no surplus value, without going to the extreme of total automation of commodities.
In my example I use the terms 'profit' and 'surplus value' interchangeably, this probably highlights my misunderstanding of the term 'surplus value'. If someone can illuminate how and why these terms are different, in so far as they don't seem to correlate AND why surplus value does therefore not increase due to automation in my example, I would also consider this a great answer.