An Oath to Finish a Gemara

Question: Someone made a Neder (oath) to learn a Massechet to be completed on the day of someone's Yahrtzeit. As the day approaches, he will only be able to complete the Gemara if he learns it quickly with a simple understanding of the words. Is it preferable to free him from the Neder or for him to learn it in the quick fashion?
Answer: He should finish the Massechet with the basic understanding since it is a great merit for the deceased, and then relearn that part in depth.