I had offer from company A and company B. I told company A that I have an offer and tried to negotiate it to get a better package. Company A asked me the offer letter, which I sent them over mail. But they couldn't give me the expected salary based on company B's offer, so they revoked my existing offer. Could company A contact company B and tell them about this, or possibly forward the email I sent tem, coz I negotiated with them at the very last moment. Will company B revoke their offer as well? Could company A do this since I negotiated at the last moment and out of their personal vendetta?
No, they won't. Unless hiring manager in A is buddies with the hiring manager in B, and they're both particularly psychotic, that's not gonna happen. It's possible but highly unlikely.
Regardless, it's not a good idea to forward your other offer letters in this way. If you withheld, you would've gotten B's counter-offer, which would likely be less than A, but you would've kept your cards close to your chest and made your decision without extra stress.