Got a lump sum relocation amount(A) and also taxes on it (amount B). Both A&B are included in my W-2 box1. Can I deduct A&B as moving expense while filing my tax return?

My company paid me a lump sum relocation amount(A) and also taxes on it (amount B). Both A&B are included in my W-2 under box 1. Can I deduct this amount as moving expense while filing my tax return ? What if my actual expense(amount C) of moving/shifting/travelling etc was less than amount A, can I still deduct amount A & B as moving expense or if I can only deduct amount C as my moving expense ? Does IRS asked for the proof of actual expense ?