If you have your cost basis for the portion of the land you are selling to your Uncle, and the selling price is less than your purchase price there will be nothing to report on the sale of the land because it is sold at a loss to a relative.
You do not have to worry about any taxes, however if you receive a tax reporting document, then you will enter it on your tax return as though there is no gain or loss (selling price and cost would be the same).
If there should be a change and a gain results on the sale, after all, then it is a taxable gain and in this scenario you would have to worry about the taxes.