Let
$(X,+)$ be an Abelian group and
$E$ be a Banach space. Suppose that
$f:X\rightarrow E$ is a surjective map satisfying the inequality
$$\begin{eqnarray}|\,\Vert f(x)-f(y)\Vert -\Vert f(x-y)\Vert \,|\leq {\it\varepsilon}\min \{\Vert f(x)-f(y)\Vert ^{p},\Vert f(x-y)\Vert ^{p}\}\end{eqnarray}$$ for some
${\it\varepsilon}>0$,
$p>1$ and for all
$x,y\in X$. We prove that
$f$ is an additive map. However, this result does not hold for
$0<p\leq 1$. As an application, we show that if
$f$ is a surjective map from a Banach space
$E$ onto a Banach space
$F$ so that for some
${\it\epsilon}>0$ and
$p>1$
$$\begin{eqnarray}|\,\Vert f(x)-f(y)\Vert -\Vert f(u)-f(v)\Vert \,|\leq {\it\epsilon}\min \{\Vert f(x)-f(y)\Vert ^{p},\Vert f(u)-f(v)\Vert ^{p}\}\end{eqnarray}$$ whenever
$\Vert x-y\Vert =\Vert u-v\Vert$, then
$f$ preserves equality of distance. Moreover, if
$\dim E\geq 2$, there exists a constant
$K\neq 0$ such that
$Kf$ is an affine isometry. This improves a result of Vogt [‘Maps which preserve equality of distance’, Studia Math.45 (1973) 43–48].