If you also offer your free book on Smashwords, Amazon will fairly soon begin offering your book free of charge. But you cannot first offer a book for free on Amazon and then elsewhere. I even wrote to them and they told me that this option is not available. I found out only later that Amazon is making comparisons with other sellers all the time. They never want to be more expensive. If, for example, you are offering your book on Smashwords for 2.99$ and you then want to lower its price on Amazon to 0.99$, you will not be able to. The price has to be 0.99$ elsewhere first, only then will Amazon give it the same price. A similar thing happens if you decide to offer your book for free on Smashwords. You will have to wait ten to fourteen days and then Amazon will offer your book for free as well.