Update records in Zoho Forms

Update records in Zoho Forms

Hi,
I'm trying to build an app which will calculate Days assigned for an user based on total duration for tasks when a Task is assigned.

following reports/ forms are used:
Task - contains subject, duration(in days), etc
User - Contains Full name, availability(for each day), email
Assignment - User Full name (lookup value from 'User'), Day1, Day2..(drop down/ lookup value of Subject from Task form)

What to achieve: 
When I assign a task to User 1 for Day, I want to subtract 1 from total duration days in Task.
When i unassign same task from same user, I want to add 1 to total duration days(current value) in Task. 

So far:
I can subtract 1 when task is assigned, but cannot add 1 to current value when unassigned.

Functions:
To Add:
void Ben.AddBen(int RequestID, int ZZZID, float NumDays)
{
    selectrequest  =  Task_Request  [ID = input.RequestID];
    selectrequest.Duration_Days = (ifnull(selectrequest.Duration_Days,0.0)  +  input.NumDays);
}

To Subtract:
void Ben.DeductBen(int RequestID, int ZZZID, float NumDays)
{
    selectrequest  =  Task_Request  [ID == input.RequestID];
    selectrequest.Duration_Days = (selectrequest.Duration_Days  -  input.NumDays);
}

OnSuccess for Assignment: (To subtract 1)
if (input.Day_1  !=  null)
{
    thisapp.Ben.DeductBen(input.Day_1, input.ID, 1.0);
}

OnValidate for Assignment: (To Add 1)
if (input.Day_1  ==  null)
{
    current = thisapp.Ben.GetCurrentRecord(input.ID); (function to get current ID)
    if (current  !=  null)
    {
        
        thisapp.Ben.AddBen(input.Day_1, input.ID, 1.0, false);
    }
}

When unassigning the task its coming up with error - "Unable to update the Value 1.0"
Error in executing AddBen workflow.
Error.
Error in executing Update Record task. Unable to update selectrequest.Duration_Days.
Unable to update the value 1.0.

Any help would be really appreciated.