What Happens when You Say Wallahi Inshallah?
Question

Four years ago, I made a vow and accompanied it by saying “In sha Allah.” So I said: “By Allah, in sha Allah, if I get a job I will give a whole month’s salary in charity.” What do I have to do now, as my salary has increased since I started this job? If I have to give charity, then should it be the amount of my salary when I started or the amount of my salary now? If I have to do that, and I want to do Hajj, my wife and I, this year, what is more important: should I give precedence to the vow or to Hajj? Please note that I have enough money for me and my wife to do Hajj, but I do not have enough to fulfil the vow and also do Hajj.
Praise be to Allah.What you said, “By Allah, in sha Allah (if Allah Wills), if I get a job I will give a whole month’s salary in charity”, comes under the heading of an oath, not a vow. If the one who swears an oath says “In sha Allah,” he will not be breaking his oath (if he does not fulfil it) and he does not have to offer expiation. The same applies to vows . So if you did not give charity, you do not have to offer expiation.
It says in Zad Al-Mustaqni‘: If someone says “In sha Allah ” in an oath that would require expiation if broken, he does not have to offer expiation even if he breaks it.”
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in his commentary: 
“What is meant by an oath that would require expiation if broken is an oath sworn by uttering the words “by Allah”, or a vow, or Dhihar [a form of divorce which was used amongst the pagan Arabs in which the man declares his wife akin to his mother by saying: “You are to me as my mother’s back”]. All of these three things require expiation if they are broken. Excluded from that is divorce and manumission of a slave; they do not require expiation if they are recanted.
So if someone says “In sha Allah ” in an oath that would require expiation if it is broken, he does not have to expiate even if he breaks it .
An example of an oath sworn by Allah is saying: “By Allah, I shall not wear this garment, in sha Allah.” If he then wears that garment, he does not have to offer expiation , because he said “In sha Allah.” And if he said: “By Allah, I shall certainly wear this garment today, in sha Allah,” then if the sun set and he did not wear it, he does not have to offer expiation .
The evidence for that is the words of the Prophet (blessings and peace of Allah be upon him): “Whoever swears an oath and says ‘In sha Allah,’ does not have to offer expiation even if he breaks it …”
An example of a vow is if someone says: “If Allah heals my sick loved one, then I vow to Allah, in sha Allah, that [I will do such and such].” He does not have to offer expiation if he does not do it. Similarly, if he says, “I vow to Allah that I will not speak to So and so, in sha Allah,” then he speaks to him, he does not have to offer expiation." (Ash-Sharh Al-Mumti‘ (15/139)
He (may Allah have mercy on him) also said: 
“If he makes a vow and says “In sha Allah,” such as if he says, “I vow to Allah that I shall do such and such, in sha Allah,” then in the case of a vow that comes under the same ruling as an oath, he does not have to offer expiation even if he breaks it.
Nonetheless, if it is a vow to do an act of worship or righteous deed, then we must examine it further. If his aim was to connect it to the will of Allah [by saying “In sha Allah”], then he does not have to offer expiation even if he breaks it, but if his aim was to affirm that he wants to do it or to seek blessing by saying “In sha Allah”, then he must do it as he intended to." (Ash-Sharh Al-Mumti`, 15/221)
The vow that comes under the same ruling as an oath is a vow by which the person aims to affirm something, deny something, prevent something or encourage something. This is called the vow of dispute and anger.
Regarding a vow to do an act of worship or righteous deed, if the person says “In sha Allah,” we must examine it further: if he intended to connect what he vowed to do to the will of Allah, then he does not have to offer any expiation. If he intended by saying “In sha Allah” simply to seek blessing or to affirm and emphasise what he was saying, then he must fulfil the vow.
We mentioned above that what you said came in the form of an oath, not a vow, so you does not have to offer expiation even if you break it.
And Allah knows best.