A/N:
I hope I have not stretched details too far. I have done my best. The story is AU after all, so I'm allowed a little creative license, right: )
Theoden considered the proposal. It was to his advantage to seek a strong ally in Denethor of Minas Tirith. And yet... He could not say he wished to send his young sister-daughter away.
"How great is the essence of forming this allegiance?"
"Very great, Theoden-King. A war between you and Gondor is imminent. If you act now, it will be avoided. Reject this offer, and Gondor will crush you."
How could he do anything else? He knew he had no choice. Rohan could not afford a war with Gondor. They would never survive.
"I will accept, Amlach. What negotiations are we to discuss?"
"The lady's dowry, for one."
Theoden stiffened. They were a poor country and had little to offer. However, he would never admit that to Gondor!
"We will give Denethor twenty-four of our finest horses and two chests of gold. We offer nothing more."
Amlach nodded. He knew well that Rohan could afford little more. Besides, he had come so close, why squabble over a dowry now?
"And what else?" Theoden continued.
"The time of their wedding. We will consider these negotiations the plighting of their troth..."
"So soon! What is the rush?" Theoden asked with suspicion.
Amlach continued patiently, "Lord Boromir and Lady Eowyn are both of age to be wed. Relations betwixt countries are more likely to fall apart if the days of the nuptials are delayed. Nay, Theoden-King, we must make haste in wedding the two. I propose that the wedding take place here in the House of Eorl in a fortnight's time. After the wedding, Boromir will take his new wife back to Gondor to live. A permanent alliance between you and Denethor, between Rohan and Gondor, will have been formed."
"Very well, Amlach. Send our acceptance back to Gondor. In a fortnight's time we will look for you."
The ambassador bowed and left. Theoden watched him go. Now to tell Eowyn...