When you're working, you ask for your boss, ''○○さん、今お手隙(おてすき)ですか？(手が空いていますか)？''

While you're doing something, you have a few second to do another thing. This situation is ''手が空いている''

Do you have time now? 今時間ありますか？=今手が空いていますか?

You can't use 手がある when you're asking someone if they're off tomorrow.