When fat is removed from the breast via liposuction, not all of the tissue removed is fat. Included in the aspirate-the material that was sucked out-will be some breast tissue. Since breast tissue has the possibility of developing a breast cancer, it would be unwise to use this tissue for a buttock augmentation.
I do not think that it is a good idea to attempt to use breast reduction tissue as donor site for fat augmentation. This tissue potentially can develop breast cancer as it does in some women.