It sounds like your "client" wants a pop-up Dialog (modal) for the Allowances.
I would suggest displaying the list of allowances per individual employee in a listbox on the form - make it scrollable so that it doesn't matter how many there are. You could also consider using a ComboBox to enter any allowances that aren't "standard" - but you might run into problems with the Revenue if you go too far away from standard terms.
Reference: Drop-down Lists & Combo Boxes (Windows)
If you do use a modal dialog then you might find this article useful (there are further article links in the article itself)
Transferring information between two forms, Part 1: Parent to Child
Finally on the UI side of things it would be better to use a Button than a label to trigger the dialog.
As to the database side of things, do NOT, I really mean DO NOT store these allowances as a single string. It is a real pain to manage them, a real pain to use them and eventually you will run out of column space to store them. It is the WORST possible design you could choose. (Have you guessed I really don't like the idea? :-) )
A better solution is to have a linking table...e.g.
Table Employee (EmpId, FirstName, LastName, etc)
Table Allowances (AllowId, Description, etc)
A table like this can link allowances to specific employees:
Create table AllowEmpl
By using this construction you can have unlimited allowances per employee, they are easily inserted, searched for, deleted and you won't have to change the database schema when more Allowances are added.