H-1B Visa Reform: What if a person holding a H1 for Company A, applies for H1 transfer through Company B, and once the transfer is approved, he changes his mind and doesn't join this company B? Instead get job from Company C and accept the job offer. Can he resign from job A and then do H1b transfer

The short answer is yes, a person is not obligated to work for B just because the company secured an approval.  But, a person must be careful about gaps in employment, which could lead to some minor violations, and a visa stamp application to be made at a US Consulate abroad.    Here is a possible fact scenario.  Foreign National (FN) works for Company A. Company B files a new H-1B.  FN stays with Company A while USCIS is processing B’s H-1B petition.  B’s petition is approved, but FN – who is still with A – decides he no longer wants B, and instead prefers C.    Company C then files a new H-1B petition for FN. FN is still in valid status with A, and also might be eligible to start with C immediately upon filing of C’s new H-1B petition (based on AC-21 H-1B portability rules).    Or FN could stay with A, and wait for USCIS approval of Company C’s petition before moving on.   Upon approval, FN leaves Company A and starts with C.     FN is not obligated to start with Company B, from an immigration perspective, simply because B secured an approval.   The facts of your situation might deviate from this, in which case it would be far easier to discuss.   Also note the commonly used phrase “H-1B transfer” can be misleading. For example, an H-1B for Company A is not really transferred to Company B, or C, at least in the sense that a person may NOT use an I-94 obtained to work for a particular company, to work for another.  Rather, each company must file and secure its own H-1B approval (I-94).   I believe the term transfer is meant to describe how an FN might quickly start working for a new company, immediately upon that company’s filing of a new H-1B petition (H-1B portability under AC-21).  But again, the reality is each company files its own petition, which ultimately has to be approved by USCIS for the FN to continue employment.   I hope this is useful, and not too confusing! Let me know if you have follow-up questions or would like to discuss logistics of the move.This answer is not a substitute for professional legal advice....

Indeed, you can elect to stay with company A. However, you might want to make sure you do not move before the H1 is approved otherwise it might create some complications.This answer is not a substitute for professional legal advice....