Abstraction of the labor contained in commodities is the basis for determining the proportions at which commodities exchange. It is a necessary component of exchange, but not identical with the act of exchange itself.

An exchange is always an exchange of use-values, for as a prerequisite, there must be inequality of use-value. A commodity is only exchanged for a different commodity, never for the same one.

The fact that abstraction is necessary to resolve the proportion at which these different use-values exchange does not eliminate the presence of use-value in the scenario. If I measure the length of the Grand Canyon in American football fields, the football field "figures" as the pure embodiment of the property length, giving expression to the length of the Grand Canyon. The Grand Canyon "figures" as itself. If length is expressed in meters, it is always a relative expression of length in reference to a meterstick, a physical object that we have set apart as embodying a standard length, never an absolute measurement.

It seems like in addition to identifying value with abstract labor, both value and abstract labor are being connected to something very different, concrete human practice.

The starting point of the analysis was concrete human practice. The real act of exchange is a form of appearance of an underlying content, which Marx identified as value. This value is deduced by the logical necessity of qualitative equivalence to resolve the quantitative proportions at which different use-values really exchange in practice.