Is the marriage assistance amount received from a relative considered a debt that must be repaid if a similar amount is not offered to him upon his marriage?

The assistance your relative gave you is not a debt that must be repaid. However, it is sunnah to reward him for it with something better, or similar, or less, according to your ability, because the Prophet, peace and blessings be upon him, used to accept gifts and reward for them. If you do not find anything to reward him with, then supplicate for him.