Money is also not something you outlaw by fiat, it’s not something you pass a law to “introduce a labour voucher system.” For Marx, money is a tool used to exchange products between decentralized enterprises, but not a tool used internally within enterprises which allocates/plans the distribution of resources directly. That means money as money gradually loses its function naturally as a consequence of the centralization/socialization of production. As enterprises grow larger and the proportion of the economy dominated by small enterprises gradually decreases, money gradually circulates less and less between separate enterprises and thus it gradually loses its function as a tool of exchange between enterprises and becomes more and more merely an internal tool of accounting.

This circumstance, then, arises from the material character of the particular labour-process, not from its social form. In the case of socialised production the money-capital is eliminated. Society distributes labour-power and means of production to the different branches of production. The producers may, for all it matters, receive paper vouchers entitling them to withdraw from the social supplies of consumer goods a quantity corresponding to their labour-time. These vouchers are not money. They do not circulate.

— Capital Vol 2

For example, the USSR had no laws introducing a labour voucher system, but they did nationalize most enterprises. This meant for the majority of cases, the ruble did not actually function as money because it did not circulate between enterprises. It functioned more as a tool of accounting within the state plan, to keep track of resources given to different state-owned enterprises. The function of the ruble as a merely a voucher, a tool of accounting, arises inevitably as a result of the gradual socialization/centralization of production independent of any sort of law to implement it, i.e. from the change in the material conditions of society as a result of the development of the forces of production.

Soviet socialism, particularly following the introduction of the first five-fear plan under Stalin in the late 1920s, introduced a new and non-capitalist mode of extraction of a surplus. This is somewhat obscured by the fact that workers were still paid ruble wages, and that money continued in use as a unit of account in the planned industries, but the social content of these ‘monetary forms’ changed drastically. Under Soviet planning, the division between the necessary and surplus portions of the social product was the result of political decisions. For the most part, goods and labour were physically allocated to enterprises by the planning authorities, who would always ensure that the enterprises had enough money to ‘pay for’ the real goods allocated to them. If an enterprise made monetary ‘losses’, and therefore had to have its money balances topped up with ‘subsidies’, that was no matter. On the other hand, possession of money as such was no guarantee of being able to get hold of real goods. By the same token, the resources going into production of consumer goods were centrally allocated. Suppose the workers won higher ruble wages: by itself this would achieve nothing, since the flow of production of consumer goods was not responsive to the monetary amount of consumer spending. Higher wages would simply mean higher prices or shortages in the shops. The rate of production of a surplus was fixed when the planners allocated resources to investment in heavy industry and to the production of consumer goods respectively.

— Toward a New Socialism

Of course, the ruble still at times did circulate, such as internationally or between the state sector and the kolkhoz sector, so it did not function purely as a voucher but that was its dominant characteristic. Dialectics teaches us not to think in puritanical terms, as Engels had written, everything in nature is connected by an infinite series of interconnected steps and there are no hard-and-fast lines between anything and as a result everything is always in gradual change to something else, containing internal contradictions from the previous and new system with it simultaneously. As Mao said, no system is pure, all systems always contain some aspects of the old and alongside some aspects of the new.

Dialectics thus defines systems not in terms of purity but in terms of their dominant characteristic. One kid having a private lemonade stand in a society that is overwhelmingly dominated by public ownership wouldn’t suddenly make it not socialist even though it is true that private enterprise contradicts with socialism. No, that kid would have to get their land and lemons from the public sector and would ultimately have to sell the lemonade back to the public sector since there would be no private buyers (only individuals with “money” provided to them from the public sector), and so the public sector would control both the lemonade stand’s supply and its demand, it would control both its input and outputs, so even though it is “private” it would still be under the control of the public sector due to that private enterprise operating in a country overwhelmingly dominated by public ownership.

The same is true in the reverse, things like public enterprise and worker co-operatives in capitalist societies dominated overwhelmingly by private capital end up taking on a capitalistic character. The reason Marxists say “socialism isn’t when the government does stuff” is not because public ownership doesn’t play a major role in socialism, it is just that public ownership in a society dominated by private capital will operate at the behest of private capital interests and thus would not have a genuine public and social character given that the state would ultimately be captured by capitalists.

Whether or not we will ever get to a point where truly speaking all money functioning as indeed money ceases to exist, some pure form of planet earth, is debatable. It’s hard to imagine that nowhere on the entire planet there isn’t any commodity being traded as a general commodity, but maybe that’s just a limit of my imagination. Although, at the end of the day, it really does not matter, because Marxism isn’t about reaching absolutely pure states of society. As long as this becomes the dominant characteristic of society, as long as money generally speaking doesn’t circulate as money but is largely just used as a tool of accounting, then we would have already achieved a moneyless society in a dialectical sense.

Since moneyless is a consequence of the centralization/socialization of production, then naturally to achieve a society where money ceases to generally circulate as money would require a society where enterprises are generally centrally planned and not decentralized on a market, i.e. it would require achieving the developed stage of socialism. You cannot get rid of money as long as you have markets, so it’s unavoidable in the socialist market economy.

It doesn’t really make much sense to implement a voucher system in a country that still is largely driven by markets because then people will be given certificates that can only be exchanged with public enterprises despite being surrounded by small commodity producers which they will not be able to trade with using those vouchers. It would be a disaster and lead to big resistance from the population, iirc I think in very early-stage Soviet Russia they did experiment with such a thing but abandoned it precisely for that reason, it had a huge amount of push-back because the vouchers were incredibly restrictive.

Personally, I don’t think it really makes economic sense to “implement vouchers” because if your country is dominated by money, that means it contains a lot of small producers and so introducing vouchers would be heavily restrictive on the population and have huge push back. However, if your economy is not dominated by small producers, then it also makes no sense to “implement vouchers” because people would already generally not be trading with small producers anyways, so the law would be redundant as you would just be implementing something by law that already is being carried out on the ground naturally.

If you want to reduce the usage of as money as money, to move more towards a voucher system, then the only real policy that should be done to achieve it is, in the words of the Manifesto: to increase the total of productive forces as rapidly as possible.