Repaying a friend who paid too much on your behalf

In the course of an ongoing business deal, Levi asked Shimon to pay a government fee on Levi’s behalf, and Levi would pay Shimon back the money paid. Shimon did so, but Levi subseqently claimed that the amount paid was more than the fee should have been. An amount was never specified in the request, but Levi claims that Shimon should have known from other discussions that the fee should have been less. Shimon claims that he just went into the government office and paid the amount that the clerk said, which is what he was asked to do by Levi. Levi now expects Shimon to resolve the issue with the government office. Shimon says that Levi owes him the money, irrespective of the confusion, since he paid it as a favor with the understanding that he would be repaid. What halachos apply? What mekorot in Chazal or Poskim apply?

The Halachic topics which apply here are as follows: To what extent is the messenger responsible in this case where he is a messenger for free. Is he obligated to pay where he corrupted the mission. In addition it is necessary to check the facts and see who is right. See the Rema Choshen Mishpat 182, 3, and ibid in Shulchan Aruch 4 and Sema 8. And Netivot Hamishpat, Chidushim 8. In this specific case we should check whether Shimon is right that it did not enter his mind that the amount is different than what the clerk asked him for or is Levi right and it is negligence on the part of Shimon as he should have known that the amount asked by the clerk is exorbitant. According to this one should rule.