Is it permissible to use a sum donated for performing 'Umrah to cover the expenses of marriage?

It is not permissible to dispose of funds donated for a specific purpose except for that purpose, or after seeking permission from the donors. This is because if a donor intends a specific action with their donation, their intention must be respected. If their intention for the donation was to assist you in performing Umrah, then it is not permissible to spend the money on anything else unless they give permission. This is what the jurists have stipulated in similar matters.