Learn English – Replace Sentence with phrase or particular word

I cannot see a single word answer that fits your exact pattern, "unless ________ I have an appointment."

But here are some ways to express the same idea.

Can we meet on Monday at 10?

That is fine with me, [but indicate the low possibility of having a conflicting appointment].

Possibilities:

• but I might/could have an appointment. [Use of italics adds stress which indicates a lower chance.]
• though there's a slim chance I have an appointment.
• assuming my unlikely appointment does not occur.
• but there's a low possibility of a conflict.
• unless, unexpectedly, the appointment I have then doesn't fall through.
• just know that I might have to cancel if a prior tentative commitment materializes.
• assuming I can clear the time, which I expect.

Feel free to mix and match, e.g., "but I might have a conflict."

Delia Smith refers to them as lumps. I bow to her authority on all things culinary.