I'm going to lie and say I made plans for the weekend I said my friend and I would finish a song.
So you do not want to finish the song then?
Yes, I'm going to do other things on that day.
You should stick to your original plan. You'll feel better after it gets finished. There is no point in lying, it will end up just hurting them.
I'm going to lie then because I don't feel like it.
I think you should either tell your friend the truth, or go ahead and finish the song. Lying to your friend is wrong and dishonest.