You decline the invitation in four paragraphs, then attach a screenshot of your calendar. Evidence. In case a plain answer can't be trusted to hold its place.
Jesus addresses the practice of swearing by heaven, earth, or Jerusalem. Each belongs to God (verses 34–35), who hears our words. A woman who knows that needs no guarantees. Her yes is yes. Her no is no. Jesus' point is truthfulness: let your plain word be true, because God hears it.
Over- explaining can come from the same root: fear that our plain word is not enough. A plain, honest answer trusts God as your defender. "No, I can't make it, but thank you for thinking of me" is a complete answer. No footnotes.
God hears your plain word and loves the woman saying it.
Let the answer carry its actual size
Jesus' teaching on oaths asks for truthful speech. The application to our yeses and noes grows from that concern. A direct answer should be something another person can rely on.
Overexplaining can hide a decision; a breezy yes can hide resentment. Neither becomes more loving because the words sound agreeable. Consider what you can honestly promise.
Practice one sentence that matches your capacity: I can help for an hour; I can't take this on; I need until tomorrow to answer. You may explain more when the relationship calls for it. Clear speech is compatible with tenderness, and a boundary should not become an excuse for contempt.
Send the answer you mean
Try a kind, accurate sentence: I can help for an hour. I cannot take this on. I need until tomorrow to answer. Add an explanation where it serves the relationship, rather than where it serves the fear.
One response to try
Give one courteous answer without inventing an excuse or disclosing more than is needed.
You can take this slowly.