Cardi B's fiance hired the powerful law firm Manatt, Phelps and Phillips, which sent Celina Powell a no-nonsense cease and desist letter, revealing she has demanded $50k and in return ... she will get an abortion.

The letter goes on to say her claim he got her pregnant is false and that she has made similar false allegations she's hooked up with other celebrities.

The lawyers say Powell's rants on social media have cost Offset great financial harm ... some businesses have pulled the plug on deals he was making with them.

The letter claims she is not only making financial demands as a money grab, she's doing it to increase her social media profile.