Her father engaged her to him, then her father died and her paternal uncles want to change the agreement
Question

                I am a 29 years old man. About two years and half ago I proposed to my work colleague. I agreed with her father about everything and we became engaged, by the will of Allah. During this period of time we had few arguments. I had a very good relationship with her father until he died last January, may Allah have mercy on him. Since the death of her father, my fiancée’s uncles have interfered in everything, and they tried to change the agreement I had with her farther. Eventually one of her uncles returned what I have bought of gold for my fiancée to me to end the engagement. Although my fiancée and I still want to continue. Some wise brothers interfered to solve this problem and both families were happy to maintain the relationship. But her uncles are stipulating illogical conditions to complete the marriage, in opposition with my previous deal with my fiancée’s father. What is the ruling of Islam on this situation? Is it permissible for her uncles to alter the stipulations of the agreement with her father? What is the solution for this stubbornness?.
Praise be to Allah.Firstly: 
Before answering your 
    question, it is essential to draw your attention to the fact that working in 
    mixed workplaces with men and women together is not permissible, and it is 
    one of the doors that lead to corruption, as its effects on society are 
    obvious. 
We have mentioned the 
    evidence for the prohibition on mixing in question no. 
    1200. 
The one who is faced with 
    the problem of working in a mixed environment – if he cannot leave that job 
    – must avoid looking at women and being alone with them and talking to them 
    about things that do not have to do with work. 
Among the negative 
    consequences of haraam mixing is what happens between men and non-mahram 
    women whom they call “work colleagues”, such as haraam looking, talking and 
    correspondence, and in many cases it leads to haraam relationships. 
Secondly: 
With regard to your 
    question, it seems from your question that you did not do the marriage 
    contract with this woman. Based on that, you are still a “stranger” 
    (non-mahram) to her so it is not permissible for you to be alone with her or 
    to talk to her too much, until the marriage contract is done between you. 
    The marriage contract with a woman is not valid unless it is done in the 
    presence of her wali (guardian). As her father has died, guardianship passes 
    to her paternal grandfather. If there is no paternal grandfather, then it 
    passes to one of her brothers. If she has no brother then guardianship 
    passes to her paternal uncles. The wali does not have the right to prevent 
    marriage without an acceptable shar’i reason. If he does prevent her, then 
    guardianship passes from him to the next closest relative, then to the 
    sharee’ah judge or one who is acting in his stead. It should also be noted 
    that the maternal uncles cannot be walis of the woman. 
See the order of 
    guardianship in the answer to question no. 
    2127. 
Shaykh al-Islam Ibn 
    Taymiyah (may Allaah have mercy on him) said: If the woman likes a man and 
    he is compatible with her, then her guardian, such as her brother then 
    paternal uncles, must marry her to him. If they prevent her from getting 
    married, then the next closest wali should give her in marriage, or the 
    ruler, without (the wali’s) permission, according to scholarly consensus. 
    The wali does not have the right to force her to marry someone she does not 
    like, or to prevent her from marrying someone she does like if he is 
    compatible, according to scholarly consensus. It is only people of ignorance 
    and wrongdoing who force women into marriage or prevent them from marrying, 
    who give their female relatives in marriage to people whom they choose for 
    their own purposes, not the woman’s interests, and force them or embarrass 
    them into doing that, and prevent them from marrying those they want out of 
    enmity towards them or to serve some purpose. All of these are acts of 
    Jaahiliyyah (ignorance), oppression and enmity, and are forbidden by Allaah 
    and His Messenger. The Muslims are unanimously agreed that they are haraam. 
    Allaah has enjoined women’s guardians to look at the woman’s interests, not 
    their own whims and desires, like all other guardians and deputies who act 
    on behalf of others. Their aim should be the interests of the one on whose 
    behalf they are acting, not their own whims and desires. This comes under 
    the heading of the trust (amaanah) that Allaah has commanded should be 
    fulfilled, as He says (interpretation of the meaning):  “Verily, Allaah 
    commands that you should render back the trusts to those, to whom they are 
    due; and that when you judge between men, you judge with justice” [al-Nisa’ 
    4:58]. This is also part of sincerity that is required. The Prophet  
    (peace and blessings of Allaah be upon him) said: “Religion is sincerity 
    (naseehah)” three times. It was asked, “To whom, O Messenger of Allaah?” He 
    said, “To Allaah, to His Book, to His Messenger, to the leaders of the 
    Muslims and to their common folk.” 
Majmoo’ al-Fataawa 
    (32/52, 53) 
But we advise you not to 
    marry her without the agreement of her family; it is essential to gain their 
    approval and win them over, so that you will not be a cause of breaking of 
    family ties that may never be healed after that.
And Allaah knows best.