wa alaykum salam wa rahmatullah,
O you who believe! Stand out firmly for Allah and be just witnesses and let not the enmity and hatred of others make you avoid justice. Be just: that is nearer to piety, and fear Allah. Verily, Allah is Well-Acquainted with what you do. (Quran 5:8)
You act and treat him by giving him his rights as a Muslim, such as returning the salam etc - Islam is perfect, it gives us our rights and responsibilities, but Muslims unfortunately are not perfect due to being humans.
You have to look out for your own rights, so you have to stand up for yourself. Often people use the deen, stooping low enough to use Islam as a way to postpone or even avoid the giving of rights.
Sakhar, a chief of a tribe, had helped the Prophet (S) greatly in the siege of Taaif, for which he (S) was naturally obliged to him. Soon after, two charges were brought against Sakhar: one by Mugheerah (RA) of illegal confinement of his (Mugheerah's) aunt and the other by Banu Saalim of forcible occupation of his spring by Sakhar. In both cases, he (S) decided against Sakhar and made him undo the wrong. [Abu Daawood]
Abdullaah Ibn Sahl (RA) was deputed to collect rent from Jews of Khaybar. His cousin Mahisah (RA) accompanied him but, on reaching Khaybar, they had separated. Abdullaah (RA) was waylaid and done to death. Mahisah (RA) reported this tragedy to the Prophet (S) but as there were no eye-witnesses to identify the guilty, he (S) did not say anything to the Jews and paid the blood-money out of the state revenues. [Al-Bukhaari]
A woman of the Makhzoom family with good connections was found guilty of theft. For the prestige of the Quraysh, some prominent people including Usaamah Ibn Zayd (RA) interceded to save her from punishment. The Prophet (S) refused to condone the crime and expressed displeasure saying:
"Many a community ruined itself in the past as they only punished the poor and ignored the offences of the exalted. By Allaah, if Muhammad's (My) daughter Faatimah would have committed theft, her hand would have been severed." [Al-Bukhaari]
The Jews, in spite of their hostility to the Prophet (S) were so impressed by his impartiality and sense of justice that they used to bring their cases to him (pbuh) and he decided them according to Jewish law. [Abu Daawood]
Once, while he (S) was distributing the spoils of war, people flocked around him and one man almost fell upon him. He (S) pushed the men with a stick causing a slight abrasion. He (S) was so sorry about this that he (pbuh) told the man that he could have his revenge, but the man said: "O messenger of Allaah, I forgive you." (Abu Daawood)
In his fatal illness, the Prophet (S) proclaimed in a concourse assembled at his house that if he (S) owed anything to anyone the person concerned could claim it; if he (S) had ever hurt anyone's person, honor or property, he (S) could have his price while he was yet in this world. A hush fell on the crowd. One man came forward to claim a few Dirhams which were paid at once. [Ibn Hishaam]