The market would need to find ways to perform the back-breaking work, either by wages increasing or the development of automation. Supply and demand. The more people who can accumulate wealth and capital, and intelligently manage that wealth, the greater the reduction in the labor supply (as they will fund their expenses using investment returns versus engaging in labor activities to do so).

Money is an abstraction, but the status quo as a whole (monetary policy, capital markets, labor policy) is designed to ensure a supply of labor. Capital begets capital [1], so those who don't have will engage in riskier behavior to get off the wage slave treadmill.

[1] https://news.ycombinator.com/item?id=16592414 ("I think capitalism is much simpler: capital begets capital. Those who have capital will accumulate more capital, almost infinitely, until the capital/income ratio reaches an equilibrium who knows how high (in the absence of major corrections like wars and hyper inflation). Add inherited wealth and low taxes. Capitalism's winners are those who already have capital. Thomas Pickety's book "Capital in the 21st Century" cogently illustrates this using massive tax return based data sets.")

Very likely much less "work" would need to be done. There are a lot of bullshit jobs that are not productive and are just making people feel bad.

Barter economies have never existed. Myth starts with Smith.

The grain still needs to be hauled to somewhere else though.

If the grain only ever exists on your farm, it has no value to anyone else whether for coins or axe heads.