Question: When Sarah gave Hagar to Abram in order to fulfill what she thought was the prophecy that she would have a child, was he committing adultery by taking Hagar or just custom?

Answer: Abraham was not committing adultery. Remember, the Torah was not given yet. There was no obligation to follow the commandments yet at all. It happened to be that Abraham was a man of truth, so he followed the Torah, which he knew inherently.

Also, the Torah does not prohibit having more than one wife. It is Rabbinically prohibited, but not forbidden from the Torah. Today, we are not allowed to have more than one wife. That did not apply to Abraham.

All the Best, Rabbi Azriel Shreiber