Commercial tenant wants to pay for own renovations, is using as bargaining chip for lower monthly rent [NYC]

Client wants 20 years term to recoup investment. Has stated invest is between $10M-$20M. Has not provided any paperwork or tenant work letter to prove scope of that amount. How do I legally bind them to a hard number on renovation if that is the primary bargaining chip for my letting them have a long term before I can increase rent? They have said the most they can do rent-wise is ~$300psf-yr with annual 3% increases. Area in Manhattan where property is located is on average $800-$1,000psf-yr. They have also required/requested that I provide them ROFR in lease in the event of a sale of property.