The gemara says that the la'av of לא ירצה which says that you can't be makriv an animal with a mum and the gemara darshened that it should include anything which is not fit to be makriv, wouldn't include a מחוסר זמן. Meaning, an animal that is premature such as before 8 days old, or אותו ואת בנו which is also considered premature, the torah is menatek l'aseh. Tosafos understands simply that the gemara means that the aseh pulls it out of the category of the la'av entirely so that one cannot be in violation of לא ירצה by being makriv an animal that is מחוסר זמן. However, Rashi seems to understand that the language of נתקו לעשה is using the standard mechanics of a לאו שניתק לעשה where the halacha is that you certainly violated the la'av but there is no malkus. The difference between rashi and tosafos is when one is makriv an animal that is מחוסר זמן, did he violate the la'av of לא ירצה - according to rashi he did, but according to tosafos he didn't. Tosafos asks on Rashi that this doesn't seem to follow the normal setup of a לאו שניתק לעשה. Normally it means that one can do something to fix up the la'av after violating the aseh such as returning the stolen goods or sending away the mother bird. Here, once you are makriv the animal prematurely (or on the same day as it's mother), there is nothing to do to fix it up. The aseh is something that should have been done instead of violating the la'av but doesn't help to "fix" the la'av. To explain Rashi's position it seems that rashi holds that a לאו הניתק לעשה doesn't have to be something active, rather it is a technicality of how the Torah presents the la'av. Rashi understands that when the Torah presents the la'av as something that can only be done after the aseh, it is a לאו שקדמו עשה and doesn't qualify as a לאו שניתק לעשה, therefore you would get malkus. But, whenever the aseh only exists after the la'av even though it doesn't fix the la'av (because once the la'av is violated, there is no ability to do the aseh), the Torah is indicating that there is no malkus for the violation of the la'av. This approach works in Rashi in our sugya and would answer the question that Tosafos has, but doesn't work in the sugya in makos of בטלו ולא בטלו and קיימו ולא קיימו which focus very much on the ability or inability to fulfill the aseh after the violation of the la'av.