Replying to professor who said that I can join his research group as a PhD student next year
He hasn't made any commitment to take you on next year. I would simply write to thank him for his consideration and say that you will re-apply if circumstances at the time permit. You don't need to say more now. 

Saying too much might be a mistake. But thanks should be expressed, certainly. Use the year to find another position or use it profitably otherwise. Reexamine your options if you are still available in the future.