There may be more or less than the $20K boot you cite. When you sell the property there will be seller capital closing costs (those that affect the taxable gain) and 1031 exchange charges which will reduce that $700K. Also, if there is still a mortgage, unless the buyer assumes it, there will be even less cash since there's probably a due on sale clause. on the exchange when you purchase that $680K replacement property there will be capital expenses (such as title charges) that will add to the cost. However, a mortgage on the replacement property would affect the cash needed or received to close the exchange and thus affect boot which would affect the taxable gain.
Ideally, if you have never done a 1031 exchange before it would be best to sit down with a pro. all the rules for a valid 1031 exchange must be met. If you mess up on just one the exchange can become fully taxable. The alternative is to work through form 8824 to see how you come out
https://www.irs.gov/forms-pubs/about-form-8824
This answer has been verified for accuracy by an Intuit expert employee
