I am working H1-B and i got an offer from a company B, i have accepted their offer and signed the required documents. Company B got my H1B transfer receipt for premium processing. Now i got the offer from another company C and i accepted their offer and i want to join the company C and not the B. I have the following queries

My joining date in company B is 18-Jan-2011 on offer letter & they ask me to join on 24-Jan so do i need to say them no before 18-jan? Any problem after saying no after 18-Jan?
will there be any legal issue if i say no to them and don't join their company?
Can there be any legal issue or problem in filling H1B transfer with company C?
Can company B sue me for any issue and ask me for visa transfer fees? Please let me know if i can have any issue in this proc?