First of all I'm confused about the differences between oaths, promises, and vows. In Qur'an it says that you should keep your promises (including promises made to God), but in some Hadiths it say that you shouldn't make vows. Please clarify this for me.
Secondly, the reason I asked the first question is that I made a promise to God that I would give a certain % of my income (well above 2.5%) to the poor if I get a job. And sure enough I got a job, but now if I do that, there is not much left for myself. I didn't take into account many of my bills. I have to help my family with money as well, could I use a portion of the money set aside to give to the poor (certain % of my income) to help my family? Is it possible to modify a promise? (for example instead of % of income I should've said % of what is left for me (after taxes, bills).
I would appreciate your answer very much, since this matter is very disturbing to me, and I don't want to break my promise. Thank you and may God watch over you.

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.

First of all, we would like to clarify the difference between oaths (Ayman) and vows (Nadhr). An oath is to swear by one of the Names of Allah or one of His Attributes to prove the correctness of something affirmatively or negatively, or to urge doing something or leaving it. Whoever makes an oath and breaks it should expiate for that by one of the following three forms of expiations: to feed ten needy persons or to clothe them or to free a person from slavery. If a person is unable to do any of the above expiations, he should then fast for three days.

As regards the vow, it is defined as a permanent or discontinued offering that the person commits himself to present to Allah if a given good thing happens to him or a given bad thing does not affect him. The vow may be made without a condition, for instance, when the person says: "It is an obligation on me to pray two Rak'a for Allah or to do I’tikaf or the like" without stating a condition for that. It is dislikable to make vows as they add more obligations on the person who might not be able to abide by them or who might do them in an incomplete manner. This is why the Prophet, the Prophet asked Muslims not to do it in a sound Hadeeth narrated by Imams Al-Bukhari and Muslim. The Prophet said: "Allah says, 'The vow does not bring for the son of Adam anything that I have not decreed for him, but his vow may coincide with what has been decided for him, and in this manner, I cause a miser to spend of his wealth. So he gives Me (spends in charity) for the fulfillment of what has been decreed for him the thing which he would not give Me before but for his vow."

But when someone makes a vow, he must fulfill it.

Allah encourages us to fulfill our vows and made this one of the characteristics of the people of Paradise. Allah Says (what means): {…and fulfill their vows.} [Quran 22:29] Allah also Says (what means): {They [are those who] fulfill [their] vows and fear a Day whose evil will be widespread.} [Quran 76:7]

Moreover, the Prophet said: "Whoever vows to be obedient to Allah, should be obedient to Him; and whoever vows to disobey Allah, should not disobey Him." [Al-Bukhari]

With regard to making a promise or a pledge, some scholars give the same rule for both the promise and the pledge whereas some others differentiate between the two. They believe that a pledge is especially about what Allah has forbidden or prescribed while a promise is for any other thing. Based on this differentiation, it is obligatory to fulfill the pledge.

As regards the promise, if it is made to fulfill something legitimate in the Sharee’ah (such as performing an act of worship or doing a favor for someone), then it is desirable to fulfill it. However, if a promise is to carry out some financial obligations or compensations, then the one who has promised it is obliged to fulfill it. For example, if someone promises someone else by saying: “if you marry that certain girl I will give you a certain amount of money”, or says “if you destroy your house I will lend you money to rebuild it”, etc, then in such conditions one is obliged to fulfill his promise.

Sometimes the statement of promise indicates that one has taken an oath or vowed to do something; in that case, the rule of oaths or vows applies on his statement.

For details about oath and vows one should refer to the books of Fiqh.

After considering the case of the questioner, we want to make clear to him that his case comes under the rules of vows; so he has to fulfill his vow. He should also continue asking Allah's Help to carry out his vow successfully.

Besides, he should keep in mind that the amount he mentioned to give in charity from his salary, on which he has taken an oath, is actually a very small amount. Indeed, such an amount should not cause any appreciable burden on a person, especially if one spends his money on that which is legal, avoiding spending in forbidden matters.

Allah knows best.