Cash-in-advance constraint

The cash-in-advance constraint, also known as the Clower constraint after American economist Robert W. Clower, is an idea used in economic theory to capture monetary phenomena. In the most basic economic models (such as the Walras model or the Arrow–Debreu model) there is no role for money, as these models are not sufficiently detailed to consider how people pay for goods, other than to say everyone has a budget constraint.

Source: Wikipedia — Cash-in-advance constraint (CC BY-SA 4.0)