With regard to debtors, Marx's sociological observations are especially important, as the debtor becomes structurally dependent upon the creditor due to the transfer of debt in the abstract. This relationship yields the concept of capital, which is central to the remainder of Marx's analysis.
So far, therefore, as his actions are a mere function of capital — endowed as capital is, in his person, with consciousness and a will — his own private consumption is a robbery perpetrated on accumulation, just as in book-keeping by double entry, the private expenditure of the capitalist is placed on the debtor side of his account against his capital. To accumulate, is to conquer the world of social wealth, to increase the mass of human beings exploited by him, and thus to extend both the direct and the indirect sway of the capitalist.— Marx, Capital Vol. I, Ch. 24, Section 3
Such bills of exchange in their turn circulate as means of payment until the day on which they fall due; and they form commercial money in the strict meaning of the term. To the extent that they ultimately balance one another by the compensation of credits and debts, they serve absolutely as money, since no transformation into actual money takes place. Just as these mutual advances of the producers and merchants to one another form the real foundation of credit, so their instrument of circulation, the bill of exchange, forms the basis of credit money proper, of bank notes, etc. These do not rest upon the circulation of money, whether it be metallic money or government paper money, but upon the circulation of bills of exchange.— Marx, Capital Vol. III, Part 5, Chapter 25
"The function of money as means of payment begins to spread out beyond the sphere of circulation of commodities. It becomes the universal material of contracts." With the advent of contracts, among other similarly related social institutions and constructs, due to fixed payments, debtors are forced to hoard money in preparation for these deadlines. It is this inequity between creditor and debtor which gives rise to an imbalance in the capitalist mode of production, and thus to class struggle. In other words, all else being equal, if production fails to grow sufficiently, the level of debt will increase, ultimately causing a breakdown of the accumulation process when debtors cannot pay creditors.
It is no longer a mere accident that capitalist and labourer confront each other in the market as buyer and seller. It is the process itself that incessantly hurls back the labourer on to the market as a vendor of his labour power, and that incessantly converts his own product into a means by which another man can purchase him.— Marx