An honest maybe is kinder than a yes that already contains resentment.

A yes you did not want is a debt that gets collected later. It buys a comfortable minute now and arrives later as lateness, quiet withdrawal, or a coldness they cannot account for, since as far as they know you agreed freely. An honest maybe is more awkward and much cheaper. It leaves them able to plan and leaves you able to mean your answer.

Say that you are not sure, and name when you will know, whenever a reluctant yes starts forming. Give the date and keep it. People adjust to an uncertain answer more easily than to an agreement that turns out to be resented.