Whilst European governments have increasingly externalised restrictive migration policies to civil actors, the latter’s main interests lie in improving or defending immigrants’ well-being. This raises the crucial question as to how civil actors deal with the puzzling position they find themselves in: to what extent do they execute or transform their funders’ policy objectives? And which mechanisms enable them to do so? This article contributes to answering these questions by detailing the historical shifts in the roles played by civil actors in the Assisted Voluntary Return programme in Belgium. Most importantly, the article argues that the considerable autonomy these civil actors achieved resulted in two seemingly opposite effects. On the one hand, they developed a wealth of expertise in ensuring the quality of return, thereby transforming the national government’s goals of managing migration into humanitarian ones. On the other hand, in recent developments their autonomy paradoxically became instrumental to migration management, not so much by changing their practices or values, but by changing their functioning within the wider field of migration policies. The article concludes by proposing the metaphor of ‘immunisation’ as an apt way of describing civil actors’ practical and functionally role in migration management.