It's really quite simple. We'll never be free if we always give away our money. Sure, money isn't real, it's not worth the paper it's printed on, the stuff that really matters are the things we exchange it for, and we can grow food free from the ground when we decide to. In the meantime, the power that money represents is the reason we were enslaved in the first place.
When Mama Araminta's bill gets printed, if you ever hand it to a cashier at Wall-Mart etc. it will be like spitting in her face.
The Black Mall and others have a campaign where they ask us to write BUY BLACK on our dollars to remind us to focus our economic power towards developing our own institutions. The only way to make this empty gesture of a tribute useful is we collectively decide to use her face to remind us, every time, to drive the extra 15 minutes, or spend a couple more seconds on the internet to find Black-owned businesses to support.
I was talking to a group of young Black boys about the importance of economic independence and one of them said to me "but I buy Black all the time, all I wear is Jordans". Harriet Tubman can free thousands more economic slaves, if only they know how much power they have.
Here's a reference list if you don't know where to start (feel free to add to it):