An oath

The men of Israel had taken an oath at Mizpah: “Not one of us will give his daughter in marriage to a Benjamite.”
² The people went to Bethel,^[a] where they sat before God until evening, raising their voices and weeping bitterly. ³ “Lord, God of Israel,” they cried, “why has this happened to Israel? Why should one tribe be missing from Israel today?”
⁴ Early the next day the people built an altar and presented burnt offerings and fellowship offerings.
⁵ Then the Israelites asked, “Who from all the tribes of Israel has failed to assemble before the Lord?” For they had taken a solemn oath that anyone who failed to assemble before the Lord at Mizpah was to be put to death.
⁶ Now the Israelites grieved for the tribe of Benjamin, their fellow Israelites. “Today one tribe is cut off from Israel,” they said. ⁷ “How can we provide wives for those who are left, since we have taken an oath by the Lord not to give them any of our daughters in marriage?” ⁸ Then they asked, “Which one of the tribes of Israel failed to assemble before the Lord at Mizpah?” They discovered that no one from Jabesh Gilead had come to the camp for the assembly. ⁹ For when they counted the people, they found that none of the people of Jabesh Gilead were there.
  
In this Chapter, we know that actually the tribe Benjamin is a tribe of Israel. God used the rest of the Israelites to punish the people from Benjamins because of their sin. And now they are doing damage control, trying to limit the damage they had done by punishing them by an invasion. What the Benajamins did to the girl(concubine) was wrong and ought to be punished. They were actually trying to find wives for the Banjamins. They went through solutions after solutions all without consulting God. We must always remember that God knows best. And simply he says: 

Matthew 1:7
"Ask and it will be given to you; seek and you will find; knock and the door will be opened to you."

So, it is a matter of asking and seeking. They could not find any solution and in the end they "gave their daughters". They did not give by agreeing to the marriage, but they "gave", allowed the Benjamins to take them as wives. This is not the way because regardless of the method, wives are given to the Benjamins.