Is it obligatory to accept substantial financial assistance offered on the occasion of marriage, or is modesty and self-sufficiency preferable?

There is no harm in accepting a favor without anticipating it, and one should repay it appropriately or supplicate for the doer; because the Prophet, peace and blessings be upon him, said: "Whoever does you a favor, repay him. If you cannot find anything to repay him with, then supplicate for him until you think that you have repaid him."