I have a friend who tried to board a plane as a Ghanian citezen to travel to australia on a tourist visa and was told by Ghana immigration service that she must first have $4000 U.S. to travel with before she could leave. Is this a true scenario

100% SCAM!!

In every country immigration Officers will never stop any outbound passengers no matter they are citizens of that country or not.

Rate of approval for Australia visitor's visas in Ghana is equivalent to zero. Where does the Australian Embassy locate? Does she really have a visa? How did she get one? She must have at least $20K in her bank account otherwise she would not be able to obtain a visa. If she has that kind of $$, she should not be asking for $4K. Etc.

Ask her all these questions... and you'll find out the truths.