Praying for the one who treats you kindly by saying “Jazak Allaahu khayran”
Question

                Is there anything wrong with saying “Jazak Allaahu khayran (may Allaah reward you with good)” to the one from whom you take money?.
Praise be to Allah.
    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.