Is there any consequence for a person who swore an oath by divorce that he would not pay a bribe to complete a transaction for his brother, and can he retract his oath if doing so would enable him to avoid the unlawful?

As long as you did not do what you swore upon—giving a bribe to this person—then there is nothing upon you. And as long as your intention with the oath was to refrain from bribery, then if you honor this person for a permissible reason, you do not break your oath by that; because intention in an oath restricts the absolute and specifies the general.