Here's a summary:
The Buffalo Bills knew of the Title IX complaint before drafting him and conducted their own vetting
Hairston volunteered for and completed a polygraph to bolster his denial .
Beane emphasized that both the school’s investigation and the Bills’ review, including the polygraph, found no evidence supporting the allegation
So, yes — he not only offered to take the test, but actually did. Whether the results will carry weight in the ongoing civil suit remains to be seen, but the team clearly considered it a critical part of their evaluation process.