الجواب حامدا و مصلیا
Working as a freelancer falls under the category of “اجیر مشترک” (Sheared Worker) in Contracts of Leasing, which is permissible according to the Sharia. If as a freelancer, one does not do any kind of work that is unacceptable according to the Sharia, then this is permissible for him and his payment will also be lawful “حلال”, under the condition that his payment, task and work period be specified.
And Allah knows best.