Upvote:4
This is almost certainly fine, especially if you have been told it is fine by a qualified immigration attorney. You are visiting the US for training and business meetings, which are permitted under the Visa Waiver Program, and will continue to be paid by your German employer, not by the US company which is your German employer's client.
I've worked for US companies with similar arrangements to hire people who live in various countries, including Germany, and it's never been a problem when they've come to visit us for a couple of weeks.