德国售后法律问题，请大侠解答--自己业余时间学习法律知识看到的题

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B with her seat in London receives a letter from S with his seat in Berlin offering her 50 used smart phones in top condition at ￡100 each for resale to be accepted within a week. Before B can send off her acceptance on this very day she receives a fax by S increasing the price to ￡120 each due to recent developments. B decides to ignore the fax and sends S an Email accepting his offer for the price of ￡100 each.
a) Has B a valid claim against S for the delivery of 50 smart phones for ￡100 each?
Provided B takes delivery of the smart phones in Germany.
b) Describe how the ownership in the smart phones passes from S to B?
c) What can S do under German law, if he discovers a week later that the purchase agree-ment was void ab initio?
Note: Germany ratified CISG,