The evolution of active mutualism through costly metabolite production is predicted to be favoured in spatially structured microbial communities, where local interactions can couple investment to reciprocal return. Central to this framework is the assumption that increased investment by a producer enhances partner growth and thereby generates a corresponding return to the producer. However, whether faster growth of partner cells translates into increased metabolite return to the producing cells remains unclear. Here, we combine single-cell microfluidics with mechanistic modelling to quantify how metabolic investment, partner growth, and reciprocal feedback are coupled in a synthetic community of amino acid auxotrophs. We show that increased investment of a primary metabolite (tryptophan) strongly enhances the growth of a dependent partner, but results in only a marginal increase in the reciprocal release of the return metabolite (proline), resulting in a net fitness cost to overproducing cells. Our results demonstrate that spatial structure alone is insufficient to favour the invasion of costly metabolic traits. Instead, the evolution of active mutualism is constrained by how metabolite return scales with investment, which can be limited by cellular physiology. When returns do not increase sufficiently with investment, costly production is not favoured, suggesting that many microbial interactions observed in nature arise from by-product release or unavoidable leakage rather than from the evolution of costly, regulated exchange.