What is the ruling on saying "God willing, you will die" or "God willing, harm will befall you" by way of jest, or saying "God willing, I will attend a certain party" when it involves sins, and can these actions constitute disbelief (kufr)?

If someone says to another, "God willing, you will die," as a statement of fact, it is true [in the sense that death is inevitable]. However, if it is said as a supplication or a wish for evil, it is not permissible. Similarly, "God willing, harm will befall you," when said as a supplication, is a blameworthy transgression in supplication. Joking with such phrases is inappropriate. Going to a place where God is disobeyed is forbidden. Whoever resolves to go to a place where God is disobeyed, knowing this, is sinful for their resolve to do evil. There is no disbelief in what was mentioned, and one must beware of obsessive whisperings (waswas) and not give in to them.