Willis Tower sells for $1.3 Billion. Such eye-popping sales headlines are fairly common today. With low interest rates and rising rental rates, we’re seeing record volume of office building sales in Chicago and many markets nationally. As part of the sale of an office building as well as refinancing, tenants will receive a document known as an “Estoppel Certificate” from their landlord as is commonly required in most office leases. Below is a brief explanation of “Estoppel Certificates” and what tenants should consider when negotiating their lease and what to look for when reviewing one.