How to deal when company is not giving relieving letter after serving notice period or buy back :

Note : my entire discussion is for dealing with “India Private IT Service Companies and Consultancies/staffing companies providing contract to hire job to their Clients/other IT Service Companies“ 1) Sometime after serving the notice period when I ask relieving letter, then they say “we can’t give you relieving letter because our client can’t relieve so don’t worry client will make you permanent after few months.” Which never happens 2) Sometime they say “it’s long procedure and lot of approvals are required to generate a relieving letter so just wait for few days”. Now in this situation if I am getting permanent offer from other company and I am not submitting relieving letter of current employer on time, due to delay they are making I may lose this new permanent job offer also. 3) Sometime they hold up the salary, if notice period is ending at the end of the month and either tell to continue job or they will not give salary and relieving letter or they will give less salary for that month Following are some of unethical things what most of the service based consultancy / staffing companies are doing 1. Not giving Salary on time, many consultancies give salary on next month mid or late mid of next month 2. Giving Less salary by mean of hidden ways to evaluate/calculate monthly salary which is against the salary figure showing in the offer letter 3. Making Spelling Mistake in Payslip with the name or number of days worked to reduce salary 4. Reducing/change Salary pay after few months and say “if you want to continue then do the job in this salary otherwise quit” or “do the job in this revised reduced salary, our client is doing this and if you quit we will abscond you and not give any relieving letter” 5. Not giving relieving letter 6. Not giving relieving letter on time 7. Threating to abscond candidate, if not doing job on those technology which never comes under candidate’s candidature 8. Firing any time when they want without informing candidate/employee, but expect from candidate to inform them before 3 to 2 months if he wants to quit. 9. Acquiring candidate’s academic mark-sheets by saying “mark-sheets are required for verification purpose” and not returning them easily, even after resigning from job I am looking clever solution (prevention is better than cure) to deal these all situations so that employer even can’t think in that direction to do such unethical ways which will destroy candidate career, his future opportunities which will impact his life. So while joining a staffing company/consultancy is there any procedure I can follow to avoid this kind of situation. When I ask companies to write one page, one paragraph statement regards my notice period policy on stamp paper they simply denied.