Jewel, I don't use DC, but I would think the reserve listing would work either way. If you decide to do it, just set up a "Reserve Listing for Customer ABC - first layaway payment" or something like that, set the amount at $100 (remember that you will have fees taken out of the price, so figure that little bit of extra in) - and assuming she purchases it on schedule, do the next Reserve listing for Dec. 1.
I'm not sure how to deal with 'customer remorse' if she decides she wants her money back rather than continuing to completely purchase - other people will probably have ideas on that.
Good luck, hope it works for you!