After John and Mary tried and approved the food, we will be using the ABC Restaurant for Peter's farewell dinner on 5 Sept at 8pm. John and Mary have ok'd the food, so Peter's farewell dinner will be at the ABC restaurant on 5 Sept at 8pm. After testing by John and Mary, the ABC restaurant will be used for Peter's farewell dinner on 5 Sept at 8pm Testing with is not correct, testing by would be grammatically correct but it doesn't sound as natural to me.
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