When it comes to support, there is always another supervisor. In your case, it sounds like at least once, you managed to get the attention of someone higher up the tree.

I would immediately follow up with those person(s) again thanking them for their help, but explaining that you need them to clear this transfer with person x.

If I owe someone money, but they give me their banking details wrong, that doesn't mean I don't owe them any more.

Also, a letter from an attorney to the right people might be effective. Not sure where to get someone who will write this for "free" though...