Leviticus 25:50

עד שנת היובל [AND HE SHALL RECKON WITH HIM…] UNTO THE YEAR OF THE JUBILEE — for after all, essentially he has bought him only to work for him till the Jubilee, for surely you must admit in the Jubilee he is to go free, as Scripture states later ,on (v. 54) “[and if he be not redeemed in these ways], then he shall go out in the year of the Jubilee”. And, of course, Scripture is speaking here of a heathen who is under your control (i. e. who has to submit to your jurisdiction, for otherwise Scripture could not dictate to him) (Sifra, Behar, Chapter 9 3; Kiddushin 16a). But nevertheless you must not practise crooked ways against him because this would result in a desecration of the Name of the Lord (Bava Kamma 113a), but when he wishes to be redeemed he must be exact in his calculation — only just as much as is due each year shall the heathen allow him as a deduction from the purchase-money: (i. e. the Jew cannot demand a larger deduction); if e. g., there were twenty years from the date when he was sold till the Jubilee and he has bought him for twenty manehs — and consequently the heathen has purchased the labour of each year for one maneh — if he (the Jewish servant) stayed with him five years and then wishes to be redeemed, he (the master) has to allow him only a deduction of five manehs from the purchase-money, so that the servant must repay him fifteen manehs. This is the meaning of: והיה כסף ממכרו במספר שנים; AND THE PRICE OF HIS SALE SHALL BE CALCULATED AS HAVING BEEN ACCORDING TO THE NUMBER OF YEARS; כימי שכיר יהיה עמו AS AT THE RATE OF THE DAYS OF AN HIRED SERVANT SHALL HE HAVE BEEN WITH HIM — i. e. he shall calculate the amount due for each year as though he had hired himself out with him e. g., at one maneh per year and that amount only he (the master) has to allow him.

וְחִשַּׁב֙ עִם־קֹנֵ֔הוּ מִשְּׁנַת֙ הִמָּ֣כְרוֹ ל֔וֹ עַ֖ד שְׁנַ֣ת הַיֹּבֵ֑ל וְהָיָ֞ה כֶּ֤סֶף מִמְכָּרוֹ֙ בְּמִסְפַּ֣ר שָׁנִ֔ים כִּימֵ֥י שָׂכִ֖יר יִהְיֶ֥ה עִמּֽוֹ׃

And he shall reckon with him who bought him from the year that he was sold to him to the year of jubilee: and the price of his sale shall be according to the number of years, according to the time of a hired servant shall it be with him.