Reply to a professor who said no open position BUT admired the work I have asked a professor in Germany for a PhD position in his lab and I sent a mail with previous work experience, future research plan, and why I want to join his group within 8-10 lines, also attached a simple CV. He replied me starting with 'You are a strong candidate' and he admired my work and plan also, but he said presently there are no open positions. 

But I want to work under his supervision. I have at most 6 months to complete my masters. How can I give a reply mentioning I have time and in that I mean time if he found any open positions I can join his lab.
You can write something along the lines of "Thanks a lot for your last mail. If you happen to have an opening in the next 3-8 months, please let me know. As already written, I would love to work with you, so you could expect an application of me in such a case. Also, if there is a graduate school offering scholarships in your department that I happen to have missed, please also let me know."

Note that pretty much the only case that you could hope for is that the professor has a currently running application for external funding that may be granted within the next few months. In that case, there may be an opening for a PhD student position (it's not really called like that, by the way) coming from that funding.