Is it permissible to accept a large sum of money as a wedding gift from relatives, or is it more appropriate to show self-restraint and contentment?

Accepting gifts and rewarding the giver is from the guidance of the Prophet, peace and blessings be upon him. He, peace and blessings be upon him, commanded us to reward those who do us a favor. If we do not find anything to reward them with, we should supplicate for them until we feel that we have rewarded them. Among the supplications is to say to him: "May Allah reward you with good" (Jazakallahu Khairan), for whoever says it has indeed conveyed the utmost praise. There is no harm in accepting a gift without eager anticipation, and one should reward it if possible with something appropriate, or supplicate for the giver.