Payment of gst vs itc

Dear Sirs,

We have to pay GST amount  "X" on account of advance received from a buyer. Secondly, we have paid GST on our purchases, Imports & under RCM- and calculated ITC available amount ,say "Y".  Whlie making payment through generating Challan at GST portal, are we required to pay-

(1) Amount X, or  

(2) Amount X-Y 

Please guide.

Please also educate-by what date tax pertaining to July month-needs to be paid !

Thanks & regards