We discuss the Isgur-Wise function (y) in the small velocity (SV) limit within the QCD sum-rule method. The behavior of (y) in the SV limit is sensitive to the particular form of the duality relations used to decontaminate the sum-rule predictions from the continuum contribution. Peculiarities of the duality relations in the problem at hand are revealed. It is shown that the proper requirements of duality and angular isotropy for S-wave states lead to an unambiguous form of the sum rules for the Isgur-Wise function. We illustrate the constraints due to these requirements using a toy model of the harmonic oscillator. The slope parameter and the shape of (y) are determined.