Sally is required to agree to the loan in writing before Mary will be allowed to receive the loan.

The requirement for Sally to agree to the loan in writing is intended to protect both Mary and Sally in the event of any issues arising out of the loan. By signing in writing, Sally is providing her guarantee that Mary can take out the loan without any issues, and will be responsible for any terms and conditions associated with the loan. This ensures that both parties understand their obligations and any potential risks. This extra protection helps to ensure that both parties are held accountable for the loan and its repayment.