How can the soundness of an act and sin coexist in a single deed, and does committing something forbidden invalidate the act?

The Usulis (legal theorists) express the issue of "separation of aspects" by stating that one and the same thing cannot be simultaneously obligatory and forbidden from a single perspective. However, it may be an act of obedience from one perspective and an act of disobedience from another. An example of this is prostration to Allah, which is an act of devotion, whereas prostration to an idol is an act of disbelief. Scholars have differed in applying this rule to certain issues, such as prayer in usurped land. The Hanbalis consider it invalid because the aspect of usurpation is not separated from the aspect of prayer, whereas the majority of scholars deem it valid, despite the sin of usurpation, due to the separation of aspects. Accordingly, whoever conceives of the separation of the aspect of the command from the aspect of the prohibition validates the action while acknowledging the sin, and whoever does not conceive of it rules the worship to be invalid.