= Solution
For $\Psi(p_1)+\phi(k_1)\to\Psi(p_2)+\phi(k_2)$, there are two <tree-level Feynman diagrams>, both with an internal $\chi$ <fermion>. Their internal <four-momenta> are $p_1+k_1$ and $p_1-k_2$. The <scattering amplitude> is
$$
i\mathcal M=(-ig)^2\bar u_\Psi(p_2)
\left[S_\chi(p_1+k_1)+S_\chi(p_1-k_2)\right]u_\Psi(p_1).
$$
These are the two possible absorption/emission orderings along the <fermion flow>. There is no third <tree-level Feynman diagram> using the cubic <scalar field> <interaction vertex>, since attaching its remaining <scalar field> line to the external $\Psi$ line would require a forbidden $\phi\bar\Psi\Psi$ <interaction vertex>.
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