“All things are requital for fire, and fire for all things, as goods for gold and gold for goods.” (K 40)
“All things are in exchange for fire, and fire for all things, just like goods for gold and gold for goods.” (L&M D87; cp. S, R, DK 90).
This fragment was read by the Stoics as providing weight to the view that fire is the Urstoff that transforms into other elements in a temporal process, ending with conflagration (Robinson, 138).  In the end all the elements that originate from fire return to fire in a cataclysmic blaze. Fire thus runs through water, earth, air (and returns to fire). In any case, fire here is symbolic of the process of change in the world and symbolic of Apollo’s lightening bolt that guides all things. The Urstoff, on the one hand, is viewed as from Apollo (thus the heavens), and as undergoing a transformation on earth, eventually though returning to fire. Yet that “(ordered) world” is also said to be made neither by god nor man but to be “an everliving fire” (Fragement 37, R 30). The aether may be a refined fire or heat of the heavens, but it is not for that created by a god. Rather it is of eternity.
The metaphor between fire and gold in this passage points to the value of fire (and perhaps the difficulty of recognizing it as the Urstoff). Recall Heraclitus’ statement in Fragment 8 “seekers of gold dig up much earth and find little.” Fire can be viewed as the common currency under all the different elements, as that hidden element of nature highlighted when Heraclitus notes that “nature loves to hide” (Fragment 10). Those who search for an understanding of nature and the process of change at work in the universe, like those who look for gold, will not find what they seek easily. But they will find something of value.
The analogy here between gold and fire as exchange is less than perfect, though. Gold doesn’t become a blanket or a lamp or a house, despite that you can buy a blanket, a lamp, or a house with it. Fire, according to Stoic view, however becomes water, earth, air. In line with this view, and in line with the idea of equal exchange, the amount of matter in the universe remains constant, even as fire transforms into the other elements.