how to account for something I already paid

I've already paid my rent for May (I pay more than I need to to build up a credit so come Christmas I have a "no rent" month).
I've written the check and it's been cashed.  But I didn't put it in the budget first, so now when I try to, it tells me that my amount available to budget is negative.  
Halp??  
(I've tried to insert a screenshot so you can see what I see.)