Is it impermissible to offer a greeting other than the greeting of Islam, such as "good morning" or "good night"? And what is the interpretation of the noble verse: "And when you are greeted with a greeting, greet in return with one better than it, or (at least) return it in a like manner. Indeed, Allah is ever, over all things, an Accountant"?

The first and best is for 'Salam' (peace) to be the initial greeting, based on the Prophet's (peace and blessings be upon him) saying: "Whoever begins with speech before Salam, do not answer him." And also his (peace and blessings be upon him) saying to Wahb ibn Umayr when he said to him: "Good morning, O Muhammad," the Prophet replied: "Allah has exchanged it for us with something better."

So, a person should begin with Salam and then follow it with another phrase like "good morning." However, if one begins with something other than Salam and suffices with it, there is no sin upon him.

As for Allah's saying in Surat An-Nisa, its meaning is: if you are greeted, then respond with something better than the greeting or similar to it. So, increasing (the response) is recommended, and equivalence is obligatory.