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German passport holders can get a 30-day visa on arrival provided they have a return ticket that shows that the person will leave Egypt within 30 days. It will totally depend on the visa officer and there is a chance that you might be denied entry in Egypt.
Your best bet is to either get a 90-day visa if you plan to use your already booked return ticket or book a new ticket with the return date within 30 days of your arrival and cancel it later if you plan to extend your stay.