I don't have any specific advise but I do have a lump that also doesn't show up on ultrasound or mammogram. I consulted two doctors and neither are too worried about it in my case. They are having me come in in three months for a recheck, probably to ease my mind since I stress about it.
However, I do worry and I hate feeling my pea sized lump that no one can seem to give me any information about, In your case, you're lucky that they will do a biopsy on you, my doctors won't because they said they may rupture my breast implants and feel my risk is low.
Good luck to you. Keep us posted on how this turns out.