Add Grid Value and Database Value together on Update/Insert

I want to create a Grid that a user can enter positive and negative values in a numeric column which are then added or subtracted to/from the original values in the database.

Is this possible?
If so how would it be done?

Any help is greatly appreciated.


A Character in a roleplayer game is earning experience, gold, copper and silver.

I select the characters in the first column from a dropdown list of all characters.

In the corresponding columns I enter values of both a positive and negative nature to either add gold etc or take it away depending on the events of the game.

Unfortunately such functionality is not available from the client-side.
The only solution is to create the required logic on your server-side.