The Hebrew man that is betrothed to the canaanite slavewomen is actually a Hebrew slave as it Says in the Midrash Sifrei on the pasuk Vayikra 19,20:במאורסת לעבד עברי הכתוב מדבר The verse is speaking about a canaanite slave who is engaged to a Hebrew slave.
The Hebrew slave is allowed to marry a Canaanite slave. This seems clear from the Verse in shemos 21,4 אִם־אֲדֹנָיו֙ יִתֶּן־ל֣וֹ אִשָּׁ֔ה וְיָלְדָה־ל֥וֹ בָנִ֖ים א֣וֹ בָנ֑וֹת הָאִשָּׁ֣ה וִילָדֶ֗יהָ תִּהְיֶה֙ לַֽאדֹנֶ֔יהָ וְה֖וּא יֵצֵ֥א בְגַפּֽוֹ׃-If the hebrew slaves master gives him a wife and she bares to him children, the wife and her children monetarily belong to the master and when the hebrew slave goes out free(after 6 years/Yovel) he goes alone.
But you might ask maybe the act is still forbidden but if the master gave him a slavewomen and he had children with the canaanite slavewoman against the will of Hashem they belong to the master?
Thats why we need the rabbis from tractate Kiddushin 15a דברים טו, יח) "כי משנה שכר שכיר עבדך"-- שכיר אינו עובד אלא ביום, עבד עברי עובד בין ביום ובין בלילה. וכי תעלה על דעתך שעבד עברי עובד בין ביום ובין בלילה?! והלא כבר נאמר (דברים טו, טז) "כי טוב לו עמך"-- עמך במאכל, עמך במשתה. וא"ר יצחק מכאן שרבו מוסר לו שפחה כנענית. Devarim 15,18: (Hashem wants you to award presents to the Hebrew slave when He leaves because) "He has worked double the regular worker work." The Gemora says a worker can only work during the day, A Jewish slave works in the day and night. But how is it possible to work at night as well (exhaustion would be not sustainable)? Surely it is written Devarim 15,16: "for it is good for him with you (the master)?" same food and drink as the master etc. Rav Yitzchak says that the work at night is that his master is allowed to force him to co habit with the canaanite slavewoman in order to have more slaves.
By the way The Hebrew slave has to do all the other Mitzvos, marrying a Canaanite slave which is normally forbidden (under the prohibition of וְלֹא יִהְיֶה קָדֵשׁ מִבְּנֵי יִשְׂרָאֵל which Targum Onkles translatesולא יסב גברא מבני ישראל אתתא אמה - A jewish person must not marry a slavewoman) is the only exception.