Annulling engagement because of a dream
Question

is it permissible to annul an engagement after two years because of the fiancé seeing a bad dream after praying istikhaarah?
Praise be to Allah.With regard to rulings and his understanding of life and circumstances, and his evaluation of the experiences he goes through, the Muslim bases all of that on the reasoning, wisdom and measures that Allah, may He be glorified and exalted, has enjoined us to pay attention to and follow. This is what Islam teaches us; it instructs us to examine, contemplate and weigh up reason and experience, then to make our decisions after that regarding people and actions. 
Allah, may He be glorified and exalted, says (interpretation of the meaning):
“Thus Allah makes clear His Ayat (Laws) to you, in order that you may understand”
[al-Baqarah 2:242]. 
There is nothing in our religion to suggest that we should rely on dreams and visions, either with regard to worldly matters or religious rulings. Knowledge that is obtained via dreams is subjective, is not reliable and there is no certainty about it. rather it is subject to doubt, and it is not possible that Islam would instruct people to refer for knowledge to sources that are imaginary and cannot meet the least academic standards. 
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: Dreams are of three types: a good dream which is glad tidings from Allah, a dream from the Shaytaan which causes distress, and a dream that comes from what a man is thinking of to himself…”
 Narrated by Muslim (2263). 
the Shaytaan plays a role in what people see in their dreams, just as a person's own self does, and it is difficult for one to be certain of the source of his dream at all times. So how can a Muslim feel comfortable with a dream that he has seen and base his decisions on it, when he knows that the Shaytaan may have played the greatest role in producing it? 
That also applies to the case of marriage. The Prophet (blessings and peace of Allah be upon him) has set out for us the characteristics on which we should base our decision of accepting or rejecting a suitor, as he said:
“If there comes to you one with whose character and religious commitment you are pleased, then marry (your female relative under your guardianship) to him, for if you do not do so, there will be tribulation and great mischief on earth.” They said: O Messenger of Allah, even if there are some reservations concerning him?? He said: “If there comes to you one with whose character and religious commitment you are pleased, then marry (your female relative under your guardianship) to him,” three times  Narrated by al-Tirmidhi (1085); he said: It is hasan ghareeb. Classed as hasan by al-Albaani in Saheeh al-Tirmidhi. 
So he defined character and religious commitment as the standards on which people should base their decisions when rejecting or accepting a suitor. Hence the questioner should not pay attention to anything else or respond to whatever she sees in her dreams of things in which the Shaytaan may have played a part, seeking to divide spouses and cause division and conflict. 
Shaykh Ibn 'Uthaymeen (may Allah have mercy on him) was asked the following question in Liqa' al-Baab al-Maftooh (no. 5/question no. 17):
A man proposed marriage to a woman, and in a dream she saw him clean-shaven. Should she agree to marry him or not? In real life he appears to be good, and he does not shave his beard; he is religiously-committed, but Allah knows best about him.
He (may Allah have mercy on him) replied:
If a woman sees a man who has proposed to her as clean-shaven in a dream, when in reality he does not shave his beard, what she sees in her dream does not matter, and that should not prevent her from marrying him, so long as he is upright in his religious commitment and character. End quote.
Moreover, we should point out that istikhaarah has nothing to do with seeing dreams – as many people think – because the purpose of istikhaarah is to ask Allah, may He be exalted, to make easier the better of two options, and turning to Him, may He be glorified, for guidance to the best of affairs. Istikhaarah is a du'aa' (supplication) and when Allah answers it, He makes easy the thing that the person has chosen, after thinking and reflecting. The du'aa' has nothing whatsoever to do with dreams.
Our advice to the sister is to think again, and not try to ruin her marital ties just because of a dream. Rather she should let religion and reason govern the issue of her engagement, then take an appropriate stance after that.
See also the answer to question no. 25793 and 34726.
And Allah knows best.