Being asked to stay late is a question. Answer it like one.

A question asked in a hopeful tone is still a question, and it is easy to hear it as an instruction when it comes from someone senior. Can you stay has an answer, and sometimes the answer is no. Treating every ask as a requirement is how extra hours become expected rather than requested. The way you answer teaches which one it was.

Next time, respond like it is a real question: not tonight, I can do Thursday. Say it in the same friendly voice you would use for anything else. Say yes when you want the hours, which is the other half of it meaning something.