All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Abstaining from eating in restaurants is a permissible matter. As for one who swears an oath to abstain from a permissible act, he has the choice to either fulfill the oath or not. If he chooses to fulfill it, he may do so, and if he chooses not to, he must offer expiation. There is no doubt that fulfilling the oath is better than violating it. Al-Buhuti, a Hanbali scholar, said in his explanation of Al-Muntaha: “The one who swears an oath regarding a permissible act is given the choice to either fulfill it or not. Fulfilling the oath is preferable, as indicated by the saying of Allah the Almighty (which means): {...And guard your oaths.} [Quran 5:89]
As for your vow (nadhr) over the oath to donate, its ruling is like the ruling of an oath. Ibn Qudamah, said in Al-Mughni: When the vow is fulfilled, it is fulfilled like an oath. Such as saying: If I speak to Zaid, then I am obliged to perform Hajj for the sake of Allah, or give charity from my wealth, or observe fasting for a year. This is an oath, and its ruling is that the person is given the choice between fulfilling what he swore to, in which case nothing is obligatory on him, or breaking the oath. In that case, he chooses between fulfilling the vowed act or offering an expiation for the oath.. and he is not obligated to fulfill it.”
Accordingly, if you violate your oath by eating in restaurants before the specified period ends, you are first required to fulfill the expiation for breaking the oath. Secondly, you have the option to either fulfill your vow or pay an additional oath expiation instead of fulfilling the vow.
Allah knows best.