This is also an example of someone who doesn't get it.
Hailee's character is not white. She is Octoroon. People who understand and know how society worked back then know of the one drop rule, etc. How you looked mattered less than your heritage. White people who didn't know her heritage didn't question her appearance. Mary was not a white woman, but was PASSING as a white woman. If you view her actions from this perspective, you understand her actions. She sees herself as black. The community that she is from knows her and accepts her as black(ish). To her, her appearance is a mask. She looks white to white people, so in her mind, she is using her mask. Because she had love for Stack and her community, she didn't want Stack to be disappointed with the low profits from opening day, so she thought she could use her "white mask" to go get some money for Stack and the community.
Sammy tells Stack at the train station that there's a white woman staring at him. Later on, after he overhears the conversation between Mary and Stack, Sammy walks over and says to Stack and says "Maybe she ain't white." Its actually funny.
The conversation between Sammy and Mary at the juke joint gave a lot of backstory. We learn that Mary's mom raised Smoke and Stack. Smoke reveals that they had actually been sending Mary's mom flowers (and probably money) the whole time all along while she was alive. We also hear Stack finally admit that he loves Mary and he set her up to live in white society so that she would have a better life.
This more than explains Mary's actions when Remmick says that he has money but gets turned around at the door. Mary's moves were out of dedication and not greed.
She loved Stack. She loved the community that she felt a part of. Nobody outside of Smoke questioned her presence at the juke joint. He knew that Stack set her up in white society with a white husband, which is why he told Stack to get her out of there. All it would take is one person identifying her as octoroon for it to all be over for her.
People see what they want to see sometimes.
Its all about perspective.