Use OCA Prepaid account to enter bills against

We have switched from a Post-Pay account to a Pre-Paid account for fuel. 

I have written the check for say $25,000.00. The account that I am using, is the OCA account. This is increasing the amount. 

We have bills entered that are coded to an expense account ("fuel") however, I need this to go against the OCA account, to reduce the amount. 

I read where you would enter a negative amount in the bill, coding it to the OCA account. 

Using the following example, this is what it does: 

     Bill is for $602.00. I changed this from the "fuel" account, as a negative amount of $602.00 to the OCA

     account in the Expenses tab.

     Once I do this and tab to the next line, it then tells me that I have to code $1,204.00 towards an expense

     account. But if I do this, it is going to double the amount for the fuel we are actually using. 

Also, how would I close out the bill as "paid" when it was pre-paid? 

This is not easy to explain. 😞