What should one do if their prayer is corrupted or they interrupt their supererogatory (nafl) prayer to catch up with the Imam: should they say the tasleem or what?

If something occurs to the worshipper that necessitates exiting the prayer, such as someone who began a supererogatory prayer and then the obligatory prayer commenced, he should simply intend to cut off the prayer and not say the tasleem, because tasleem is the act of concluding the prayer. However, one whose prayer has become invalid should depart without tasleem or intention, due to its invalidation.