Question regarding Miles one way cash return

Hello,
I was rethinking upcoming hols
Now thinking about using my miles to get 4 UC flights to mia (there is availability) and then paying cash to get 4 Y's back on the day flight from jfk 3 weeks later.
On reading the combo fares page its states that I cant use combo to get UC flights. but its a bit obtuse to my mind.
Does that mean I cant pay cash for upper back or does it mean I cant combine upper on miles and y cash. Its simply not clear to me!
So can I go upper miles out and return Y cash or not?
ta.
I was rethinking upcoming hols

Now thinking about using my miles to get 4 UC flights to mia (there is availability) and then paying cash to get 4 Y's back on the day flight from jfk 3 weeks later.
On reading the combo fares page its states that I cant use combo to get UC flights. but its a bit obtuse to my mind.
Does that mean I cant pay cash for upper back or does it mean I cant combine upper on miles and y cash. Its simply not clear to me!
So can I go upper miles out and return Y cash or not?
ta.