Why did Fiona marry Barclay?

Was watching this on Turner...had to step out of the room and missed the part where she lures the George Brent character (Mr. Barclay)...don't therefore understand her motivation for doing so, trying to get him to propose marriage, when she didn't even like him, and in fact on their wedding night gave him $25k, I think it was, for him to agree to end the marriage...? What was Fiona's reason for wanting to marry him? It didn't have anything to do with the mansion/court battle, did it -- I thought Barclay wasn't involved in the charity until later?