Let $K$ be a compact Lie group acting in a Hamiltonian way on a symplectic manifold $(M,\Omega )$ which is pre-quantized by a Kostant-Souriau line bundle. We suppose here that the moment map $\Phi$ is proper so that the reduced space $M_{\mu }:={\Phi }^{-1}(K·\mu )/K$ is compact for all $\mu$. Then, we can define the “formal geometric quantization” of $M$ as$${𝒬}_K^{-\infty }\left(M\right):=\sum _{\mu \in \widehat{K}}𝒬\left(M_{\mu }\right)V_{\mu }^K.$$The aim of this article is to study the functorial properties of the assignment $(M,K)\to {𝒬}_K^{-\infty }\left(M\right)$.