Good manners dictate that you should reciprocate the one who does you a favour. The Prophet (Peace And Blessings Of Allaah Be Upon Him) enjoined that and said: "Whoever does you a favour, then reciprocate, and if you cannot find anything with which to reciprocate, then pray for him until you think that you have reciprocated him." Narrated by Abu Dawood (1672); classed as saheeh by al-Albaani.

Reciprocating kindness means choosing that which will make the one who did you a favour feel happy, just as he made you feel happy. So you should strive to thank him in kind. Allaah says (interpretation of the meaning):

"Is there any reward for good other than good?"

[al-Rahmaan 55:60]

If you are not able to reciprocate with a gift, or helping with some task, or offering some service, etc, then at least you can say du'aa' for him, and this du'aa' may be a means of bringing him happiness in this world and in the Hereafter.

One of the best forms of du'aa' for the one who has done you a favour is that which is mentioned in the Sunnah:

It was narrated that Usaamah ibn Zayd (may Allaah be pleased with him) said: The Messenger of Allaah (Peace And Blessings Of Allaah Be Upon Him) said: "Whoever has a favour done for him and says to the one who did it, 'Jazak Allaahu khayran,' has done enough to thank him."

Narrated by al-Tirmidhi (1958) and by al-Nasaa'i in al-Sunan al-Kubra (6/53); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.

This du'aa' is also narrated from the Prophet (Peace And Blessings Of Allaah Be Upon Him) in a lengthy hadeeth in which it says: "You are the Ansaar, may Allaah reward you with good (jazaakum Allaahu khayran), for you are decent and patient." Narrated by Ibn Hibbaan (7277) and al-Haakim (4/79). He said: its isnaad is saheeh, and al-Dhahabi agreed with him. Shaykh al-Albaani said in al-Silsilah al-Saheehah (3096): It is as they said.

This phrase was often uttered by the Sahaabah (may Allaah be pleased with them).

It says in Musannaf Ibn Abi Shaybah (5/322): 'Umar ibn al-Khattaab (may Allaah be pleased with him) said: If one of you knew what there is in his saying to his brother, 'Jazaak Allaahu khayran', you would say it a great deal to one another.

Usayd ibn al-Hudayr (may Allaah be pleased with him) said to 'Aa'ishah (may Allaah be pleased with her): May Allaah reward you with good (O 'Aa'ishah), for by Allaah, you never have any problem but Allaah grants you a way out and makes it a blessing for the Muslims. Narrated by al-Bukhaari (336) and Muslim (367).

In Saheeh Muslim (1823) it is narrated that Ibn 'Umar (may Allaah be pleased with him) said: I came to my father when he was wounded and they praised him and said: May Allaah reward you with good, and he said: I have hope and fear – meaning hope for that which is with Allaah of reward and mercy, and fear of that which is with Him of punishment.

What is meant by the phrase "Jazaak Allaahu khayran (May Allaah reward with you good)" is asking Allaah to reward you with a great deal of good.

Fayd al-Qadeer (1/410)

Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) said in Sharh Riyaadh al-Saaliheen (4/22):

Reciprocation depends on the situation. With some people, reciprocation may be by giving him something similar to what he gave you, or more. With others it may be by praying for him and he would not accept a gift of money, because if a great man who has a lot of wealth and high status and honour among his people gives you something and you give him a similar gift, he will think that you are failing to give him his due, but for such a person you should pray to Allaah for him. "And if you cannot find anything with which to reciprocate, then pray for him until you think that you have reciprocated him." That may be by saying to him, Jazaak Allaahu khayran. That is because if Allaah rewards him with good, that will bring him happiness in this world and in the Hereafter. End quote.

And Allaah knows best.