I saw a rashash today that I should have included in my sefer Nasiach B'chukecha. Tosafos asks that rather than forcing the master to free the chatzi eved to fulfill the mitzvah of sheves, we should allow him as is to marry a regular woman because the mitzvah of pru u'rvu should push off the issur of lo yihyeh kadesh? Tosafos gives 3 answers: 1. Since we can force him to free the slave it is possible to fulfill both and therefore we don't allow d'chiya. 2. It is not simultaneous because p'ru u'rvu is only done at g'mar biah. 3. If he would marry a bas chorin or shifcha she would be violating an issur without a mitzvah, and even if she is commanded in sheves (although not in p'ru u'rvu) she can marry someone like her.
The Maharsha asks on the first answer, why don't we say that the mitzvah of "l'olam bahem ta'avodu" which is an issur aseh not to free a slave, should push off the issur of lo yihyeh kadesh. Meaning that when we want the mitzvah of pru u'rvu to push of lo yihyeh kadesh, we should not consider freeing the slave an option since it itself can push off the issur? The Rashash answers, that we never find an issur aseh that is achieved by NOT doing something, can push off an issur b'kum v'aseh. Interesting yesod but I don't know if it fully answers the question. The question of the maharsha can still be that although the mitzvah of l'olam bahem ta'avodu in and of itself is not able to push off an issur, it at least should not be considered an option. When the mitzvah of p'ru u'rvu comes to push off lo yihyeh kadesh, and we want to say that it can't because there is an option to free the slave, that should not be considered an option since it would entail the violation of an issur. Obviously this question is only according to those who disagree with the Ran back on 38b and consider this a real mitzvas aseh that doesn't disappear even when not being done for the sake of the eved.