Ghazali also notes that there might also be a problem of one person not even needing what the other has to offer, but this is almost an afterthought; for him, the real problem is conceptual. How do you compare two things with no common qualities? His conclusion: it can only be done by comparing both to a third thing with no qualities at all. For this reason, he explains, God created dinars and dirhams, coins made out of gold and silver, two metals that are otherwise no good for anything:

Dirhams and dinars are not created for any particular purpose; they are useless by themselves; they are just like stones. They are created to circulate from hand to hand, to govern and to facilitate transactions. They are symbols to know the value and grades of goods.

They can be symbols, units of measure, because of this very lack of usefulness, indeed lack of any particular feature other than value:

A thing can only be exactly linked to other things if it has no particular special form or feature of its own for example, a mirror that has no color can reflect all colors. The same is the case with money—it has no purpose of its own, but it serves as medium for the purpose of exchanging goods."
From this it also follows that lending money at interest must be illegitimate, since it means using money as an end in itself: "Money is not created to earn money." In fact, he says, "in relation to other goods, dirhams and dinars are like prepositions in a sentence," words that, as the grammarians inform us, are used to give meaning to other words, but can only do so because they have no meaning in themselves.

Money is thus a unit of measure that provides a means of assessing the value of goods, but also one that operates as such only if it stays in constant motion. To enter into monetary transactions in order to obtain even more money, even if it's a matter of M-C-M', let alone M-M', would be, according to Ghazali, the equivalent of kidnapping a postman.