Nedarim 8 cites R’ Gidal’s halacha that a person can accept an oath as a motivator to mitzvah performance, e.g. the gemara uses the example 'ashkim v’ashaneh perek zeh', a vow that 'I shall arise and learn this perek'. The Rosh writes that this oath is a neder, similar to a neder which one makes to give tzedaka. However, the Ran argues and writes that R’ Gidal’s oath is a shevua, because a neder can never take the form of ‘kum v’aseh’. This fits well with the Ramban discussed yesterday who argues that nidrei bituei can never be b’kum v’aseh. Why, however, did the Ran (like the Rosh) not categorize this vow as nidrei gavoha, just like giving tzedaka, which can be forumulated b'kum v'aseh? Based again on the Ramban, we can draw a distinction. Tzedaka creates a monetary shibud, an obligation, similar to hekdesh. Promising to learn a perek of gemara is a personal obligation, but has no dinei mamomos component. Therefore, it falls under the domain of nidrei reshus and not nidrei gavoha. (Based on R’ Reichman’s Reshimos Shiurim of R’ Soloveitchik).
This lomdus rests on the assumption that a pledge to tzedaka is not just a commitment to give money, but the pledge transfers ownership of the money to the poor. This is a major issue in Rishonim - stay tuned.