Am I held accountable for anything if I intend to fast a makeup day for Ramadan on a specific day, but then do not fast it and postpone the makeup?

If the intention was not made before Fajr, then your fast is valid, and breaking it is not permissible unless there is a valid excuse. Some scholars are of the opinion that it is obligatory to make up the day that was intentionally broken during the Ramadan makeup, and this is a view held by the Malikis. However, expiation (kaffarah) is not required in this case.

If you made the intention to break the fast before Fajr, then there is no blame on you for postponing the makeup if there is sufficient time before the next Ramadan. However, if only a number of days equal to the makeup days remain, then it is not permissible to postpone it. And whoever does so intentionally will be liable for the expiation for delaying for each day.