If Matthew has his way, he won't be using the inheritance to save Downton. Unfortunately, I don't know whether Matthew will get his way. Fact is, we don't know whether Matthew will inherit Downton. He's the heir presumptive, not the heir apparent. (Sorry Eug.
) Cora could die and Robert could marry another younger woman who could have a son and Matthew would lose out on being the Earl. Even if Cora doesn't die, Matthew could die without having sired a son and then the estate would be entailed away to another distant Crawley male (perhaps the chimney sweep Violet talked about in 2x04). I surely hope that if this does occur, that Robert and Violet won't be forcing Mary to marry this man or the next heir could already be married and have a son. It would in fact, be prudent for Matthew to buy Downton from Robert with his new inheritance and whether or not the title continues with Matthew and Mary's progeny, the house would stay with them.
There's also a number of things I don't understand about this inheritance that I've already said somewhere else on
(though i don't quite recall where) That Reggie Swire would have decided to leave money to someone who also succumbed to the Spanish Flu like Lavinia. If you're planning to leave the money to someone, wouldn't you have been invited to his funeral or at least notified of his death? JF could have decided to that the fellow died of a heart attack a month before Reggie Swire did but Reggie didn't know because he himself was on death's door.