A friend deserves an honest response before an uncertain yes turns into a late cancellation.

A hesitant yes is usually kindness in the short term and a cost in the long one. It buys you a comfortable moment and leaves the other person planning around something you were never sure of. When it collapses two hours before, they have lost the evening and the alternative they could have arranged. The uncertainty was always there; only its disclosure was delayed.

When you are asked and you do not know, say that instead of saying yes. Give a day by which you will know for certain. Most people would rather hear a real maybe on Monday than an apology on Friday night.