Need to join in the new employer but leaved in probation period

I had 4 years and 7 month of experience in a company, After that I had attend the interview with two MNC. Let take two company names as A and B. First I got offered from the A company. So I accepted it and joined the company. After Joining, within 10 days, I have received the better offer from the B company (FYI: My A company probation period is 3 month and notice period in probation period is 1 month) and also I have informed about the A company joining to the B company. They told me to join with the relieving letter or acceptance letter for my resignation. So I have put the resignation letter to A company & they have relieved me within 20 days since I don't have any work to handover but they told me I should paid that 10 days amount. I also agree with A company. While leaving A company, they don't provide any letter format for acceptance letter for resignation (but they provide no due clearance form with the relieving date) and they told me they will provide the relieving letter after 15 days. And I also informed the B company that I will get the relieving letter after 15 days and also I have send that no due clearance form as proof (but B company has refuse to accept that no due clearance form as proof they need only letter format). After 15 days of relieving A company, they asked me to paid the amount. I also paid it. But it's been 20 days after relieving. I didn't get the relieving letter. when I ask the HR, he simply replying that, he will provide the relieving letter ASAP. I have proof (phone conversation and E-Mails). B company is said they will reject the offer if they get delayed. Here are my question: 1. Is there any law to provide the letter format while leaving the office by the employer to employee. If so, please provide the section and rules for it? 2. Is there any possible way to get the relieving letter soon? 3. I also have doubt that B company will reject my offer. What I can able to do if they reject my offer.