As far as I can tell, you are supposed to enter an estimated amount on Line 18 of Schedule E.  So if $10,000 was earned for the entire year (Line 3), maybe $7000-ish was earned during that time period that you should enter on Line 18.

Then if the client does NOT want to actually defer the amount (I generally recommend not to), then you can go to the 1040Wks, to Line 12e of Schedule 3.  You can change Line 11 of that worksheet to $0 if the client does not want to defer that amount.