That isn't how a 1031 exchange works. You can't defer the gain by paying the principle of rental property you currently own.
Basically, you would have to purchase a replacement property for the property you sold in order to defer the gain on the sale (within a defined time period).
You can read about the section 1031 IRS code in the link below.
https://www.irs.gov/uac/like-kind-exchanges-under-irc-code-section-1031
http://www.1031exchange.com/faq/
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