I.
It was narrated by Abu Dawud (3272) and by Ibn Hibban in Al-Ihsan (10/197) and others from `Amr ibn Shu`ayb, from Sa`id ibn al-Musayyib, that two brothers of the Ansar had an inheritance between them. One of them said to his companion: (Let us) divide it. He said: If you ask me again to divide it, my wealth will be at the door of the Ka`bah (i.e., donated to be spent on it). `Umar said to him: The Ka`bah has no need of your wealth; offer expiation for your oath and speak to your brother. I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “No oath or vow is binding on you if it involves disobedience to the Lord or severing the ties of kinship or if it involves something over which you have no control.”
There are some scholarly reservations about the isnad of this report, as it was said that Sa`id ibn al-Musayyib did not hear directly from ‘Umar, so the isnad is interrupted (munqati`).
A number of the hadith scholars were of the view that its isnad is intact, and that Ibn al-Musayyib did indeed hear from `Umar, or it is almost intact because Sa`id specialised in the fatwas of `Umar (may Allah be pleased with him) and paid a great deal of attention to them, even if he did not hear them directly from him.
Ibn al-Qayyim (may Allah have mercy on him) said:
Imam Ahmad and other leading scholars said: The reports of Sa`id ibn al-Musayyib, from `Umar constitute proof in our view. Ahmad said: If we do not accept the narration of Sa`id from `Umar, then whose narration should we accept? He saw `Umar and heard from him. This was stated by Ibn Abi Hatim.
So his reports from him are not interrupted according to what Ahmad said.
Even if there is some interruption [in the chain of narration], this interruption has no impact according to the leading scholars, because Sa`id is the most knowledgeable of people about the fatwas of `Umar, and ‘Umar’s son `Abdullah would ask Sa`id about them.
If Sa`id ibn al-Musayyib narrates a mursal report from the Messenger of Allah (blessings and peace of Allah be upon him), it is accepted, so how about if he narrates from `Umar?
End quote from Tahdhib Sunan Abi Dawud, 2/401-402.
And it was narrated from Sahabah other than `Umar that they issued fatwas similar to his [on this matter].
Imam Malik narrated in Al-Muwatta’ (2/481) from `A’ishah the Mother of the Believers that she was asked about a man who said: I will place my wealth in front of the door of the Ka`bah, and `A’ishah said: He should offer that which will expiate for his oath.
Al-Hafiz ibn Hajar (may Allah have mercy on him) said:
It was narrated by Malik and al-Bayhaqi with a an authentic (sahih) isnad. Ibn as-Sakan also classed it as authentic.
End quote from At-Talkhis al-Habir, 6/3126.
II.
The man’s words “… my wealth in front of the door of the Ka`bah” meant that he was committing himself to give his wealth in charity for [the upkeep of] the Ka`bah.
Al-Khattabi (may Allah have mercy on him) said:
When he mentioned the door of the Ka`bah, he did not mean that he was literally going to place his wealth in front of its door; rather what he meant was that he was going to donate his wealth to the Ka`bah, or towards the costs of the Kiswah (cover) of the Ka`bah, or towards its maintenance and the like.
End quote from Ma`alim as-Sunan, 4/48.
The context of these words indicates that the one who said that intended to prevent himself from upholding ties of kinship with his brother and dividing the estate between them, so this was a vow intended as an oath.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Al-Athram said: `Abdullah ibn Raja` told us: `Imran informed us, from Qatadah, from Zurarah ibn Abi Awfa, that a woman asked Ibn `Abbas about a woman who vowed that if she put on her garment, she would give it in charity. Ibn `Abbas said: Did she say that in anger or otherwise? They said: In anger. He said: Allah, may He be Blessed and Exalted, does not accept a righteous deed done in anger; let her offer expiation for her oath.
I say: Ibn `Abbas asked about the motivation for the oath: did she intend to draw close to Allah by what she vowed to do, such as when a person says: If my wealth is saved, I will give it in charity? Or was it her intention to swear that she would never wear that garment, so she has to offer expiration for breaking her oath? So he said: Was it in anger or otherwise? And when they said that it was in anger, he knew that what she said was akin to an oath and not a vow. Hence the jurists call this “a vow made in anger,” so it is in fact an oath, even if it sounded like penalising oneself.
End quote from Majmu` al-Fatawa, 35/340.
The point is:
`Umar told him that he had to offer the expiation for breaking an oath, reconcile with his brother and uphold ties of kinship. This was stated by many of the scholars.
Al-Baghawi (may Allah have mercy on him) said:
The scholars differed regarding a vow if it sounds like an oath, such as if someone says: “If I speak to So and so, I promise Allah that I will free a slave” or “If I enter the house, I promise Allah that I will fast or pray.” This is a vow that sounds like an oath, because the intention is to prevent himself from doing that action, like the one who swears an oath intending thereby to prevent himself from doing a particular action. Therefore many of the scholars among the Sahabah and those who came after them were of the view that if he does that action, he must offer expiration for breaking an oath, just as if he broke his oath. This is the view of `Umar and `A’ishah; it is also the view of al-Hasan and Tawus, and of ash-Shafa`i according to the more correct of his views, and of Ahmad and Is-haq.
End quote from Sharh as-Sunnah, 10/36.
Ibn al-Mundhir (may Allah have mercy on him) said:
What I say is that the one who swore to give his wealth in charity, or said: “I will donate my wealth in Allah’s cause if I do such and such,” if he breaks his oath, he must offer expiation for breaking his oath, because Allah, may He be Glorified and Exalted, says: {Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families} [Al-Ma’idah 5:89]. Moreover, it is soundly narrated from the Messenger of Allah (blessings and peace of Allah be upon him) that he said: “Whoever swears an oath, then realises that something else is better than it, let him offer expiation for his oath and do that which is better.”
No one has the right to regard anything that sounds like an oath as not being an oath except on the basis of evidence from the Quran or Sunnah or scholarly consensus.
End quote from Al-Awsat, 12/115.
III.
In your case, your telling your friend “I will leave” is merely a statement, and there is nothing in the context of your words to indicate that you made it binding on yourself to do something by means of an oath or a vow. Rather it was just a threat that you were going to leave.
Based on that, you do not have to do anything, because the expiation in such cases is only required in the case of binding oaths, and in this instance there is no phrase which would indicate that it was an oath or a vow.
And Allah knows best.