Donation deductions

I manage a "for Profit" thrift store. A customer made a donation and would like me to sign a paper acknowledging that her donations are worth $90,000. I think her estimated value seems inflated. We are a very small business and our yearly sales are roughly $30,000 at best.   I'm scared to sign something that acknowledges that we received  $90,000 dollars worth of items when that is triple of our yearly sales. Will I get in trouble with the IRS or trigger an audit  if i sign the paper?

Thank you for taking the time to read/answer my question.

John