Is the seller in that much of a hurry? If not then just explain to him what you just said here, about the weather not cooperating, and you don't want to either get stuck out there or tear up his yard, he should appreciate that and work with you. But as far as draining the calcium chloride, I'd just pull the core out of the stem with the valve down low and let er rip. If they are still full, the tubes can't be that shot or it would have leaked out by now.