You can't prove that. They took a masking agent for the sole purpose of hiding steroid usage.
They took tests after the failed tests to prove they had no steroids in their system. There are many ways to test, I find it hard to believe that this masking agent covers you from every type of test and I find it hard to believe they would waste time taking a secondary test that proves nothing, surely they were subsequently tested using a method that the Bumetanide can not mask.
Still doesn't change the fact that the NFL knew players used this substance and that the NFL knew the substance had substances in it that were not listed and were not allowed and they did not inform players, they did not put it on their banned supplements list despite their knowledge.