Marx states, [emphasis mine]

The character of creditor, or of debtor, results here from the simple circulation. The change in the form of that circulation stamps buyer and seller with this new die. At first, therefore, these new parts are just as transient and alternating as those of seller and buyer, and are in turns played by the same actors. But the opposition is not nearly so pleasant, and is far more capable of crystallisation. The same characters can, however, be assumed independently of the circulation of commodities. The class-struggles of the ancient world took the form chiefly of a contest between debtors and creditors, which in Rome ended in the ruin of the plebeian debtors. They were displaced by slaves. In the middle ages the contest ended with the ruin of the feudal debtors, who lost their political power together with the economic basis on which it was established. Nevertheless, the money relation of debtor and creditor that existed at these two periods reflected only the deeper-lying antagonism between the general economic conditions of existence of the classes in question.

Does, then, this imply also a relationship in capitalist society where debt and finance play a role in the proletarianization of the so-called middle-class (or petty-bourgeois)? After all, you start a small business to jump into self-employment or to make others work for you, and when you inevitably endebt yourself (your business) to achieve success, are you not at risk of proletarianization if the business fails due to unpayable debt?

Or is the force of the haute bourgeoisie a greater factor in proletarianization, thus making the difference of the petty-bourgeois business owner being in debt or not aside from Marx's argument? Surely Marx would've remarked, at some point or another in his writings, the relevance of this if it was actually central to capitalism.