The core passage at Bava Kamma 93a records Rabbi Yochanan's statement that there is a 'yes' that functions as a 'no' and a 'no' that functions as a 'yes,' illustrated by two baraita cases: one who says 'strike me, wound me' and answers 'yes' when asked if it is on condition of exemption is treated as though he said 'no,' while one who says 'tear my garment' and answers 'no' to the same question is treated as though he said 'yes.'
On this same passage, Rashi (Bava Kamma 93a:14-16) frames the distinction in terms of the verse 'to guard,' explaining that a bailee is liable only when the article was given to him to guard and not when it was handed over on condition that it be destroyed or distributed, since in that case there is no longer anyone who can be said to have deposited it with him.
Tosafot (Bava Kamma 93a:12) raises a difficulty with Rashi's reading of why the rule differs between the opening and closing clauses of the mishnah, and offers its own distinction: in the case of bodily injury, even an affirmative answer is presumed to be said in jest (sarcastically), while in the case of monetary damage, a negative answer is likewise presumed to be sarcastic, because the one who was asked had already been told to tear the garment or break the jug.
The sources provided do not include a passage of the Rambam addressing this specific statement of Rabbi Yochanan on Bava Kamma 93a about 'yesh hen k'lav'; the Mishneh Torah material available here concerns different matters entirely, such as the law of increase in value of stolen property after despair in Mishneh Torah, Robbery and Lost Property 2:4-9 and the law of accepting charity from a gentile in Mishneh Torah, Gifts to the Poor 8:9-10, neither of which bears on the exemption-condition dispute at hand.
Several other sugyot preserve distinct statements made in the name of Rabbi Yochanan that are unrelated to the 93a dispute: his teaching that one who robs another of a peruta's worth is considered to have taken his life in Bava Kamma 119a, the discussion of leaving produce for the poor and acquisition by change of form in Bava Kamma 94a and Tosafot on Bava Kamma 94a:9, the ruling on reciting a blessing over a newly purchased vessel in Berakhot 60a, his comment on the day the Temple burned in Taanit 29a, his ruling on sukkah covering made of flax in Sukkah 12b, and his position on idolatrous festivals cited by Lechem Mishneh on Mishneh Torah, Foreign Worship and Customs of the Nations 9:5:3; none of these address the 'hen'/'lav' exemption language of 93a.
Likewise, the teaching on the severity of theft from a gentile recorded in Tosefta Bava Kamma 10:8 stands apart from the 93a discussion and does not supply an explanation of Rabbi Yochanan's statement there.