וחשב עם קונהו, "he will make a reckoning with the one who purchased him.⁠" The verse warns that one must make a meticulous accounting and not try and trick the Gentile in one's calculations. The reason is that stealing from pagans is prohibited as a form of desecrating the holy name of the Lord. The Torah demands such careful accounting when the Gentile in question is under Jewish authority. I might have thought that seeing the Torah permits charging interest to Gentiles, something which the Torah elsewhere appears to view as a form of unethical conduct, that stealing, at least indirectly, from a pagan is permitted; therefore the Torah makes a point of telling us that this is not so. We know already from Joseph's brothers whose money had been returned to them by the Egyptians (at least they thought so) that they took it back to Egypt with them (Genesis 43,13) in response to their father Yaakov's instructions. If Yaakov thought (as he said) that even if the pagans had made an error we must not take advantage of it to their detriment, how much more so must we not disadvantage them deliberately. All of this occurred before the Torah was given, when we did not have these limitations imposed upon us. How much more meticulous must one deal with a Gentile now that the laws of the Torah apply to us! The prohibition is derived from Deut.7,16 that "during the time God gives us the land of the seven Canaanite tribes we are commanded to destroy them.⁠" The words אשר אני נותן לכם, "which I am about to give to you,⁠" make this commandment an exception. Only the pagan nations God gives to us may be treated in such a fashion as described in that verse. We are to relate to other Gentile nations in accordance with the moral and ethical imperatives which are part of the Jewish code of ethics.
Another law derived from the words in this verse that one is to make a careful calculation is that the redeemer must calculate the number of years remaining in the contract of the Jewish slave to the Gentile before the Yovel year, and he must pay him for the years of service that the Jew will not serve him. The basis of the law is to prevent the desecration of the Lord's name, i.e. not to give the pagan an opportunity to claim that our God deals unfairly with him who had paid good money for the services of this slave. To allow such a thought to take hold in the mind of a Gentile is a more serious crime than doing the same thing to a Jew.
The Tosephta Baba Kama 113 states expressly that if one has robbed a pagan one must return the stolen goods. It adds that such robbery is a graver crime than robbing a Jew because the crime includes the sin of desecrating the Lord's name. When one robs a fellow Jew the victim does not use this as an excuse to question the rules of the Torah and the fairness of God.
You will find an interesting illustration of this principle in the inaugural prayer of Solomon when the Temple was dedicated. Solomon prayed that when a Gentile comes all the way from across the ocean or from countries closer by in order to offer his prayers to the Jewish God in the Temple, that God should respond positively to such prayer. Regarding similar prayers offered up by Jews, Solomon asked that if the Jews in question were worthy then God should respond positively to such prayers. Clearly, the reason that Solomon asked for a blanket acceptance of prayers offered by the Gentiles was in order not to give them an opportunity to denigrate the Jewish God which would have been a form of desecrating the Lord's name (compare Kings I 8,39-43 respectively). Solomon was not afraid that if a Jew's prayer would not be answered positively, that such a Jew would question the existence or fairness of the Lord as a result.