What is the ruling on someone who works for only one month to receive the salary then resigns; is this permissible?

An 'Ijarah (leasing/hiring) contract is binding on both parties, and neither party may unilaterally terminate it except with the consent of the other, or due to the presence of a defect.

If the contract has a fixed term, it is not permissible to terminate it before the term expires. Some scholars are of the view that one who terminates a fixed-term contract is not entitled to any wages, while the majority of scholars hold that such a person is entitled to wages for the work performed.

If the contract has no fixed term, but renews monthly, then it is permissible to terminate it after the end of each month. Leaving the work after the end of the month constitutes a termination of the contract.