It is said that haman sold himself to mordechai when he was broke. Did haman become a jew like a eved kenaani that is mechuav bemitzvos keisha.I have heard different opinions. This was brought up in the gemara about teviloh of avadim.Some say that haman become enraged " asher higid lahem asher hu yehudi "means that they were calling haman a jew because of his ownership by mordechai ( the rav in the shull said this was the CHASAM SOFER)
(1) I looked up the Chasam Sofer you cited. It is in Toras Moshe - Shir Ma'on - to Megilas Esther 3:4. The Chasam Sofer indeed writes that Haman was an "Eved Kena'ani" and had been circumcised and was obligated in Mitzvos like a woman.
The Chasam Sofer cites the Gemara in Megilah 13a that the name "Yehudi" is a general name. It seems that he is referring to what the Gemara says there that anyone who denies idol-worship is called a Yehudi". Therefore even though Haman was an Eved Kena'ani, he was still called a Jew, because since he was obligated in some of the Mitzvos this should mean that he had abandoned idol-worship.
The Chasam Sofer explains the verse (Esther 3:5) "And Haman saw that Mordecai did not bow down to him", to mean that, on the contrary, every time Mordecai saw Haman in the Gate of the King, he showed him the deed of sale which proved that Haman was Mordecai's slave, and this is why Haman was filled with anger. (See Megilah 15a).
(2) However I did find different explanations. Talelei Oros (to Megilas Esther 5:13, p.202) cites Admor Mi'Piltz who asks - since Haman was Mordecai's Eved, why did he not circumcise him?
Admor Mi-Piltz gives an ingenious answer. Haman was descended from Amalek, who was the son of Eisav ie. Edom. The word Edom means red. The Gemara in Shabbos 134a cites Abaye that if a baby is red this suggests that his blood has not yet been absorbed in his limbs and therefore it is dangerous to circumcise him, so one has to wait until the blood is absorbed. However Eisav and his descendants always remain red so they can never be circumcised. See Teshuvas Achiezer 4:45 that a potential convert who is incapable of undergoing a Bris because of danger, can never become Jewish.
[However Mordecai was permitted to keep him as a slave because our Gemara (48b) states that if one buys a non-circumcised Eved on condition that he will not be circumcised, it is permissible to keep him].
(3) Talelei Oros (p. 203) also cites the Gerer Rebbe ZT'L, Rabbi Avraham Mordecai, who answered on the basis of our Gemara (48b) that if one owns an Eved Kena'ani and he does not want to have a circumcision, one may give him 12 months to decide while one tries to persuade him, and this is what happened with Mordecai and Haman.