This should do it:
My_DICT = {'Sale_Type' : 'Car', 'Sale_Length' : 5, "Query_String" : ""}
My_DICT['Query_String'] = "select * from Sales where Length > %s" % My_DICT['Sale_Length']
— EDIT —
Considering your input example, you have a list of dictionaries in My_DICT. You could do it like this:
for element in My_DICT:
element['Query_String'] = "select * from Sales where Length > %s" % element['Sale_Length']
5
solved Variable in Dictionary Value String Python