Gopal Balakrishnan posted these notes by Marx on Facebook. I’d never come across them before but they feature a good comparison between Neo-Ricardian socialist theories of value and Marx’s monetary theory of value:
Since labour-time is the substance and the inherent measure of value, the names thus indeed express the value relations themselves. In other words it is asserted that labour-time is the real standard of money. Here we leave the Birmingham school and merely note in passing that the doctrine of the ideal measure of money has gained new importance in connection with the controversy over the convertibility or non-convertibility of bank-notes. While the denomination of paper is based on gold or silver, the convertibility of the note, i.e., its exchangeability for gold or silver, remains an economic law regardless of what juridical law may say. For instance, a Prussian paper thaler, although legally inconvertible, would immediately depreciate if in everyday commerce it were worth less than a silver thaler, that is if it were not convertible in practice. The consistent advocates of inconvertible paper money in Britain, therefore, had recourse to the ideal standard of money. If the denominations of money, pound, shilling and so on, are names for a determinate amount of particles of value, of which sometimes more, sometimes less are either absorbed or lost by a commodity when it is exchanged for other commodities, then the value of an English £5 note, for instance, is just as little affected by its relation to gold as by its relation to iron and cotton. Since its designation would no longer equate the bank-note in theory to a determinate quantity of gold or of any other commodity, its very concept would preclude the demand for its convertibility, that is for its equation in practice with a determinate quantity of a specific thing.
John Gray was the first to set forth the theory that labour-time is the direct measure of money in a systematic way.  He proposes that a national central bank should ascertain through its branches the labour-time expended in the production of various commodities. In exchange for the commodity, the producer would receive an official certificate of its value, i.e., a receipt for as much labour-time as his commodity contains,  and this bank-note of one labour week, one labour day, one labour hour, etc., would serve at the same time as an order to the bank to hand over an equivalent in any of the other commodities stored in its warehouses.  This is the basic principle, which is scrupulously worked out in detail and modelled throughout on existing English institutions. Gray says that under this system
“to sell for money may be rendered, at all times, precisely as easy as it now is to buy with money, … production would become the uniform and never-failing cause of demand.”
The precious metals would lose their “privileged” position in comparison with other commodities and
“take their proper place in the market beside butter and eggs, and cloth and calico, and then the value of the precious metals will concern us just as little … as the value of the diamond.”
“Shall we retain our fictitious standard of value, gold, and thus keep the productive resources of the country in bondage? or, shall we resort to the natural standard of value, labour, and thereby set our productive resources free?”
Since labour-time is the intrinsic measure of value, why use another extraneous standard as well? Why is exchange-value transformed into price? Why is the value of all commodities computed in terms of an exclusive commodity, which thus becomes the adequate expression of exchange-value, i.e., money? This was the problem which Gray had to solve. But instead of solving it, he assumed that commodities could be directly compared with one another as products of social labour. But they are only comparable as the things they are. Commodities are the direct products of isolated independent individual kinds of labour, and through their alienation in the course of individual exchange they must prove that they are general social labour, in other words, on the basis of commodity production, labour becomes social labour only as a result of the universal alienation of individual kinds of labour. But as Gray presupposes that the labour-time contained in commodities is immediately social labour-time, he presupposes that it is communal labour-time or labour-time of directly associated individuals. In that case, it would indeed be impossible for a specific commodity, such as gold or silver, to confront other commodities as the incarnation of universal labour and exchange-value would not be turned into price; but neither would use-value be turned into exchange-value and the product into a commodity, and thus the very basis of bourgeois production would be abolished. But this is by no means what Gray had in mind – goods are to be produced as commodities but not exchanged as commodities. Gray entrusts the realisation of this pious wish to a national bank. On the one hand, society in the shape of the bank makes the individuals independent of the conditions of private exchange, and, on the other hand, it causes them to continue to produce on the basis of private exchange. Although Gray merely wants “to reform” the money evolved by commodity exchange, he is compelled by the intrinsic logic of the subject-matter to repudiate one condition of bourgeois production after another. Thus he turns capital into national capital,  and land into national property  and if his bank is examined carefully it will be seen that it not only receives commodities with one hand and issues certificates for labour supplied with the other, but that it directs production itself. In his last work, Lectures on Money, in which Gray seeks timidly to present his labour money as a purely bourgeois reform, he gets tangled up in even more flagrant absurdities.
Every commodity is immediately money; this is Gray’s thesis which he derives from his incomplete and hence incorrect analysis of commodities. The “organic” project of “labour money” and “national bank” and “warehouses” is merely a fantasy in which a dogma is made to appear as a law of universal validity. The dogma that a commodity is immediately money or that the particular labour of a private individual contained in it is immediately social labour, does not of course become true because a bank believes in it and conducts its operations in accordance with this dogma. On the contrary, bankruptcy would in such a case fulfil the function of practical criticism. The fact that labour money is a pseudo-economic term, which denotes the pious wish to get rid of money, and together with money to get rid of exchange-value, and with exchange-value to get rid of commodities, and with commodities to get rid of the bourgeois mode of production, – this fact, which remains concealed in Gray’s work and of which Gray himself was not aware, has been bluntly expressed by several British socialists, some of whom wrote earlier than Gray and others later.  But it was left to M. Proudhon and his school to declare seriously that the degradation of money and the exaltation of commodities was the essence of socialism and thereby to reduce socialism to an elementary misunderstanding of the inevitable correlation existing between commodities and money.