Hi Kristyna,
I've done something quite similar to this in the past! In this case you don't actually need to use cursors, just the field calculator using Python will do! If you right click on the third field (BELOW) and go to "calculate field", select "python" for the expression type, put the following code in the code block:
def myfunc(field1, field2):
if str(field1) in field2:
return field1
else:
return None
Then in the box above the codeblock, put the following line
my_func(!NADR_GID!, !ABOVE!)
It should do exactly what you're hoping for it to do. Basically taking the first number, converting it to a string, looking for that sequence in the second field, then returning the first field's value if there is a match!