Is donating a quarter of one's salary considered an arduous and unnecessary commitment, or is it a positive act that contributes to society?

If the covenant was merely an intention and not uttered, then nothing is required of you. However, if you uttered it, then it is safer to consider it a vow that must be fulfilled, as preferred by Sheikh al-Islam Ibn Taymiyyah. You are rewarded for the good you do, and it is hoped that Allah will reward you in this world and the Hereafter, in accordance with His saying, the Almighty: "And whatever you spend of anything, He will replace it; and He is the best of providers."

Regarding the ruling on initiating a vow, the preponderant opinion is that what is disliked is the vow of recompense (contingent on a condition), whereas the vow of devotion (pure and unconditional) is not disliked. Therefore, what you have undertaken is permissible. The scholar Al-Shinqiti elaborated on the ruling concerning the initiation of a vow, explaining that the prohibition only applies to a vow contingent on the attainment of a benefit for the vower, while a pure vow of closeness (to Allah) is among that which has been encouraged and its fulfillers praised.