Imladris I would have to say that they are interchangeable. A form of Saruman's chance of manipulation is what he tried to do with Gandalf. Trying to have Gandalf join him, that is trying to manipulate him. The Ring, yes it offers what the person wishes, or desires, but think of this scenario.
Boromir, can be argued as a good person, a strong leader, and in heart he is a good person, he means to do good. The Ring appeals to him, shows him able to gain power, prestige...etc. That is also how I would define manipulation, where Boromir, a person who means to do the right thing, is persuaded, and falls to the Ring's will.
I will agree with you, that brainwashing and manipulation are totally different, that was a bad choice of words by me. However, I think if the Ring offers what a person want, that could still be manipulation. Boromir wants power, he tries to resist the Ring, but can't do it, he was persuaded to do otherwise.
Or maybe, manipulation is too strong a word, maybe "persuaded" will fit better. I hope that cleared some things up
.