Hello Plarium,
Today I received the -50% Defensive Unit Recovering from the Discount Generator. Unfortunately it doesn't work. There are sufficient diamonds to recover units. 
Hello! Thanks for the report.
Developers are aware of this issue.
Note that the discount works for Defense only, while you are trying to restore Offense.
The error should not appear while restoring the corresponding type of units.
Good luck!
I tried to recover Defense units and it showed me the same error.Hello! Thanks for the report.
Developers are aware of this issue.
Note that the discount works for Defense only, while you are trying to restore Offense.
The error should not appear while restoring the corresponding type of units.
Good luck!
Thank you for the clarification!
The issue will be resolved in the future updates. We apologize for the inconveniences.
