An interesting question is being discussed by various poskim.

The question is - someone in New York, an older single fellow, asked his friend to travel on his behalf to Eretz Yisrael and daven for him by th e grave of Rabbi Shimon bar Yochai in Meron, so that he should find his bashert. In return, he promised to pay him for the service.

For whatever reason, the friend changed his plans without discussing it first with his buddy in New York and instead of traveling to Eretz Yisrael and davening by the Rashbi, he traveled instead ot Lizhensk in Poland on the yahrtzeit of the Noam Elimelech and davened there for his friend to find his soulmate.

Shortly after, his friend actually found the woman of his dreams and they engaged to be married.

When he returned from his trip he went to his friend and asked for the money he had been promised. The chosson refused to pay up, sayign that he had appointed him to go to Meron and daven by Kever Rashbi, not to go to Lizhensk, and therefore the task he had hired him for was never done, and he is not obligtaed to pay.

Rav Zilbershtein relalted to this issue and said the resolution depend son what the chosson had claimed - if he says he only believes in the ability of Rashbi to provide such salvations as the Arizal and others testified about the power of the Rashbi, but not by other kivrei tzaddikim and he does not believe the salvation came form the prayers offered on his behalf in Lizhensk, then he does not have to pay, as he specifically wanted the Rashbi. However, if he also believes in the power of the Noam Elimelech and agrees that it is possible the salvation had come from those prayers, but claims that the messenger did not do as he was employed to do, then he has to pay.

Other rabbonim, specifically Rav Mordechai Gross, Rav Yaakov Meir Stern, and Rav Moshe Shaul Klein, argue and say he cannot be obligtaed to pay, as the messenger did not fulfill his obligations. And, even if he does believe in the powers of kivrei tzaddikim like that of the Noam Elimelech to work its magical powers and send the shidduch, he still does not have to pay, as he still might believe there is a difference between kever Rashbi and other kivrei tzaddikim, as he specifically wanted kever Rashbi. As well, the power given the response via the channel of the Noam Elimelech is different than that provided via the channel of the Rashbi, and he specifically wanted the influence of the Rashbi.

Others, such as Rav Shmuel Eliezer Stern and Rav Shamai Gross, said he has to pay. They reason that there is no gaurantee by davening at any grave, even by kever Rashbi. Yet, since the prayers were clearly answered, it should not matter to him which grave the fellow was at. And sure if the messenger had spoken toh im first and said he'd prefer to got ot the kever of the Noam Elimelech instead, he for sure would have agreed, as he just wants to already have his shidduch.
source: Behadrei

1. interesting debate
2. It is clear that they say that it is a matter of what the guy actually believes about what is happening. Meaning, it is in each person's mind, and that somehow affects reality. One person might believe, and the next person might not, and each would get different results because of it. And if he believes at a more micro level, not just grave sin general but also which specific grave has more power, that also affects his reality.
3. The last argument quoted is strange. it is a bold assumption to assume that he would have been ok with a change in plans because he does not really care, just wants results. I am not sure why they are so sure they can assume that.