I've been a contractor/consultant for years - I'm familiar with the issues tax authorities have across Europe with regards to consultants being considered as an employee or as a contractor but I have until just now, I have never heard of the term "arbeitnehmerueberlassung" (AUN or AUG - I cannot recall the acronym).
If recruited under a arbeitnehmerueberlassung contract, does this contract have any limits (in duration) ? does it include health, pension, unemployment (arbeitlose) contributions, holiday and sick leave?