Fath al-Rabbani

Benefits: … Imam al-Albani rahimahullah says: Abu Amr al-Shaybani said: The meaning of "la ghirar" is that a person should not exit his prayer while suspecting that any part of the prayer remains; rather, (he should give salam) when he is certain that the prayer is complete. Ibn Athir said: By "ghirar al-salah" is meant to be deficient in the qualities and pillars of the prayer, and by "ghirar al-taslim" is meant that the worshipper responds (to a greeting) by saying "wa 'alayk." Imam al-Albani rahimahullah said: The meaning of "wa la taslim" is not that a non-praying person should not greet the one praying with salam, because it is established in several hadiths that the noble Companions used to greet the Prophet sallallahu alayhi wa sallam with salam (and he sallallahu alayhi wa sallam would respond with a gesture). As our master Abdullah ibn Umar radi Allahu anhu narrates, once the Messenger of Allah sallallahu alayhi wa sallam went to the mosque of Quba and was praying, and during that time, some Ansar came and greeted him with salam, even though he was in prayer. I asked our master Bilal: When these people would greet the Prophet sallallahu alayhi wa sallam with salam while he was in prayer, how would he respond to them? He said: He (would gesture) like this. Then (to describe the manner of his hand), our master Anas radi Allahu anhu spread his palm forward. (Abu Dawud) (Sahihah: 318)

In summary, until the worshipper is certain of the completion of his prayer, he cannot give salam (to exit the prayer), and he also cannot verbally respond to a greeting, because that is considered speech, which is forbidden in prayer. (And Allah knows best what is correct)