In the Currency and NumericInput controls, clear the sign of the value on SelectAll in order to allow the user to enter a negative value again. At the moment when everything in the controls is selected, the minus key only toggles the sign of the value. Instead you should be able to select a negative value and replace it with another negative value by entering the minus key and the new value.

MinusKeyCustomizations demo is added in our SDK: https://github.com/telerik/xaml-sdk/tree/master/MaskedInput/MinusKeyCustomizations