It is perfectly legal--no law prohibits an employer from asking this. And while technically you do not "have" to answer, if you choose to not answer, the employer is free to not hire you or even to terminate the interview right then and there; candidates who will not provide information which prospective employees may legally request do not have to be hired.If you lie about the number and that lie is later discovered, you have committed fraud--not only could you be terminated, but under some circumstances, the employer could also sue you for costs or damages it incurred due to your lie. For example, if it paid you a higher salary based on your lie than you would have received, it might be able to recover some portion of that; it could also recover recruiter fees if the only reason it hired you (and incurred those fees) was due to your lie.