I have consulted the dictionary.
Do we use "resign" only when a person leaves their job at will? He resigned from his job.
Do we use "sack" if the boss decided on his resignation? He was sacked for not obeying the company rules.
Do we use "resign" only when a person leaves their job at will? He resigned from his job. Yes.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Do we use "resign" only when a person leaves their job at will? He resigned from his job. Yes. Another term is eg He quit his job.
Do we use "sack" if the boss decided on his resignation? He was sacked for not obeying the company rules. Yes, although I think that is British English. In N. America, I hear eg He was fired / let go / terminated.