Regarding that particular couple from the OP, there are numerous ways used to siphon money into developing countries but so much of it never reaches the people it is meant to help. There is a lot of guilt being heaped on the heads of decent people.
Christians can be very susceptible to that kind of persuasion and there are those who exploit it. We have sent billions to "developing nations" but very little changes. Has Bob Geldof manged to feed the world yet? There is so much corruption.
Obviously that congregation was taken in.
Having said that I don't want to get too judgemental about all the prosperity preachers, they do have to answer to God. Who am I to say God hasn't blessed some people for reasons I don't know. I don't know what each one does with their money.
I think we should take notice of this though, Jesus says in Revelation 3
14 “To the angel of the church in Laodicea write:
These are the words of the Amen, the faithful and true witness, the ruler of God’s creation. 15 I know your deeds,that you are neither cold nor hot. I wish you were either one or the other! 16 So, because you are lukewarm—neither hot nor cold—I am about to spit you out of my mouth. 17 You say, ‘I am rich; I have acquired wealth and do not need a thing.’ But you do not realize that you are wretched, pitiful, poor, blind and naked. 18 I counsel you to buy from me gold refined in the fire, so you can become rich; and white clothes to wear, so you can cover your shameful nakedness; and salve to put on your eyes, so you can see.
We can't all be lavishly wealthy obviously. If The Lord made all Christians rich he would have a kingdom full of people who just there for the money!