Selling a model created with a mixamo model

Hi I'm working on a model for someone that has a modified mixamo head as part of the main model, which they intend to sell later on. The head has been heavily modified to the point it does not look like the original, and is attached to the model (it is not separate.) I was wondering if this would be against the license? If it's okay to sell, do I need to credit the original creators?