Assalaamu alaykum. I got married last week and on the day of the marriage, the uncle came to me and quoted a false hadith arguing that the bridal gift should be an amount enough to cause a bit of pain to the groom (for security purposes). Even though I insisted that the amount should be such that I start my married life without debts, he refused, stating that his sources were strong. Due to the lack of time for verification and to avoid a conflict that could result in bad blood between either sides, I agreed and the bridal gift was set at 85,000 SAR (this may be irrelevant but the average bridal gift may be around 6,000 SAR or in the close vicinity). I only had SAR 17,000 with me in-person, from which I planned to give the bridal gift as well as some few gold ornaments in order to please her. If Allah permits and grants me life, I should be able to pay the bridal gift in a few years time, but I fear that it may effect our relationship as what I spend on my wife would be felt as an obligation (as part of the bridal gift with her permission) rather than out of compassion as my spouse. I believe that the bride and her folks are very understanding and can be convinced, backed by Quran and Sunnah, but having signed the contract:
Is it lawful for me to approach the guardians and request them to reduce the bridal gift to make it easier for me? If they refuse, is it lawful for me to approach my bride to request the same?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If the woman is adult and mature, then she is the one who decides the issue of her bridal gift, not her guardians or any others, because the bridal gift is her exclusive right. Allaah, The Exalted, says (what means): {And give the women (upon marriage) their (bridal) gifts graciously. But if they give up willingly to you anything of it, then take it in satisfaction and ease.} [Quran 4:4] If she wishes, she may give up her right to all of it or part of it.

Ibn Qudaamah wrote:

"It is allowable for the woman to give up her right to the bridal gift or part of it, exempting her husband from that obligation, or take it in her possession and then offer it to him as a gift, and if she is competent to dispose of her money (i.e. she is a sane adult), then her actions are valid. We do not know of any difference of opinion among scholars in this regard. Allaah, The Exalted, says (what means): {...unless they forego the right...} [Quran 2:237] The pronoun 'they' in the verse refers to the wives. Allaah, The Exalted, also says (what means): {But if they give up willingly to you anything of it, then take it in satisfaction and ease.} [Quran 4:4]..." [Al-Mughni]

Al-Qurtubi wrote, "The scholars agreed that if a woman who is legally competent gives up her bridal gift and willingly offers it to her husband as a gift, then it is effective and she is no longer entitled to it." [Tafseer Al-Qurtubi]

Hence, there is no harm in talking with your wife so as to reach a mutual understanding about giving up her right to part of her bridal gift; and she has the right to accept or refuse your request in this regard.

It should be noted that the Islamic Sharee'ah encouraged reducing the bridal gift as one of the means to have one's marriage blessed. It has been narrated by ‘Aa'ishah that the Prophet, sallallaahu ‘alayhi wa sallam, said, "The best woman is the one whose bridal gift is the easiest to pay." [Ahmad]

For more benefit, please refer to fatwa 106818.

As for what was mentioned in the question, that the amount of the bridal gift must be big enough to cause a bit of pain to the groom (for security purposes), then this is a false claim, and it is contrary to what we have explained above.

Allaah knows best.