if type('This.cFieldSource') <b>= 'C'</b> and !empty(This.cFieldSource)
   This.Value = eval(This.cFieldSource)
endif

In the LostFocus, you have to have:

if type('This.cFieldSource') <b>= 'C'</b> and !empty(This.cFieldSource)
   replace (This.cFieldSource) with This.value
endif