If you are eligible for Medicare because of a disability, End-Stage Renal Disease (ESRD) or Amyotrophic Lateral Sclerosis (ALS), you are eligible for premium-free Part A.
If you return to work, as long as you remain medically disabled you can keep your Medicare coverage indefinitely. For the first 8.5 years you will not have to pay the Part A premium. After that, you will have to pay the Part A premium yourself. If at that point your income is too low to afford the Part A premium, there is a program called the Qualified Disabled and Working Individuals (QDWI) program that can help.