Is this a send or receive transaction?
It sounds like you may have created a transaction a long time ago that was never carried out. Make sure you are using the latest version of Dash (currently 0.12.2.2) and go to Tools > Wallet Repair > Recover transactions 1/2. Try 1 first, then 2 if it didn't work. After that try Rescan blockchain files. This should make sure there is no discrepancy between what your wallet thinks it holds or has sent/received, and what the blockchain says.