Variable on an interface

Hi Readers,

Needed an insight regarding the below.

I need a variable holding a single value, which can be updated through an action when a form is submitted; But it need to be visible in the form but as read only.

What i have tried: I have kept a read only grid to make it as read only, instead of a temp variable i have queried the data from a table and made only single row is visible so that the last updated value is visible.

But i do not want the value to save  in a table and query it to make it visible on the form,
instead i just need a temp variable which holds the last updated value as read only.

Elaborated:

I Am trying to create an application which has a read only variable on the form called wallet balance();

Application: There will be an interface/form which has wallet balance, current value and operation and a submit button;

We enter the wallet balance by seeing it in the read only grid, we enter the value which need to be added/deducted from wallet and an operation(+ or -) is choose and submit the form;
The wallet balance which is read only need to be updated based on the operation choose weather it can be adding to wallet balance or deducting from wallet balance.

Use case: I have said what i have tried above; but i want only the last updated row need to be visible i would like to know how can we do that;

and instead of entering the wallet balance manually it need to take the last updated row as wallet balance and calculate based on our input; and the last updated row need to be visible always as it is the current wallet balance

I should not get this rounded one i just need to get only the value