The fact that $\overline{AS}$ is a median is irrelevant here. You're asking if, given lengths $AB$ and $AS$, you can determine (uniquely) the triangle $\triangle ABS$. There are infinitely many such triangles — imagine fixing $\overline{AS}$ and drawing a circle of radius $c=8$ centered at $A$. All but two of the points on that circle can be vertex $B$.
The fact that $\overline{AS}$ is a median is irrelevant here. You're asking if, given lengths $AB$ and $AS$, you can determine (uniquely) the triangle $\triangle ABS$. There are infinitely many such triangles — imagine fixing $\overline{AS}$ and drawing a circle of radius $c=8$ centered at $A$. All but two of the points on that circle can be vertex $B$.