1031 money to pay off mortgage ?

[AS Chow]

Sorry if asked before, but this just came up in conversation and I think I know the answer, but not 100%.

If for example I get 700k from a building (A) that I want to use 1031 Exchange and buy a building (b) that costs 2m, and I get a mortgage for 1.3m can I then sell another building (c) next year for 500k and take that money to pay off part of the 1.3m mortgage on building (b) ?

Assume the 700k and 500k is all Cap Gains and those are the total proceeds I receive from building A & C.  

(I'm a small owner in several buildings that are getting sold and I don't really have any control of, or know exactly when they're going to be sold.)

Thanks !