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T. Szostok and Sz. Wąsowicz in (Szostok, Wąsowicz, 2011) studied the following functional inequality: $|F\left(y\right)-F\left(x\right)-\left(y-x\right)f\left(\frac{x+y}{2}\right)|\le \epsilon$ stemming from the Lagrange mean value theorem. They proved that the functon $f$ is affine, provided $f,F:ℝ\to ℝ$ satisfy the above inequality for all $x,y\in ℝ$. The aim of our paper is to extend the results of (Szostok, Wąsowicz, 2011) to more general situations (for example, we change $ℝ$ to $ℂ$ or $ℍ)$.