Keep promises in smaller form when circumstances cannot support their largest form.

A promise made in good conditions often cannot be delivered in bad ones, and people tend to treat that as a binary. Either the full thing happens or the promise is broken. There is usually a smaller honest version available, and offering it is far closer to keeping your word than disappearing while you wait for capacity to return.

When you cannot deliver what you said, say so early and offer the reduced form specifically. Half an hour instead of the afternoon, a short note instead of the long one. People forgive shrinking far more easily than silence.