ahahahahah....are you serious or you are joking bro?
selling DF21 to Saudi arabia is to break U.S monopoly?
Do you know that it is the U.S who gave both Saudi Arabia and China the green light for this sale to take place?
If they U.S had said No to this deal, then it would never have happened in the first place, Same with the Turkish missile defence system you are trying to sell Turkey, if the U.S is really against the deal, then im sure it won't go through believe me.
You seem to underestimate the leverage/influence the U.S has over all arab middle eastern countries(bar a few stubborn ones like Iran, Syria), even with big China the U.S has lots of leverage/influence to persuade/pressure it to sanction its ally Iran talk much less of little Arab states.
So i advice you to look for a better excuse to Iran for the missile sale to the Saudis.