Adding a Value to a column in sharepoint list

[Filistyn]

Hello!

I have this sharepoint list:

I am working on time off app and column "VacationValue" is a value, that sets number of free days each employee has. Every manager can see their staff's time off requests in a gallery. Manager can either accept the request or decline. My question is, if he declines, how to add back those Vacation days, since I have already abated them from the VacationValue value. I have tried this on the decline button: 

Patch(Employees, {VacationValue: VacationValue + (ThisItem.EndDate - ThisItem.StartDate)})

But it does not work.

Someone help please

[Filistyn]

I just realized, that there is a problem. The gallery is a list of a different sharepoint list

So the button wont work, for which item in the first database he should those d

[Filistyn · Accepted solution]

Nevermind I have solved it with this formula:

Patch(Employees, LookUp(Employees, NameOfEmployee = ThisItem.Title), {VacationValue: LookUp(
      Employees,
      NameOfEmployee = ThisItem.Title
   ).VacationValue + ThisItem.LenghtInDays});