It was the price he paid to leave that time hole that he and Sayorian were stuck in. But by giving the price he is imortal as long he doesn't leave the shop. There is another reason why he can't leave but that one is more complicated to explain.
I'll check Holic I haven't read it yet, but Tsubasa they could have ended it better. How are Fay and Kurogane supposed to get back to their own worlds? What about the other worlds? Have all the people in Clow country died because of But-Chin's time loop? How did Yuko get that tube? Too many questions, not enough answers.