Entering rebates for closed quarters

Hello everyone.  I have a small business in California and we purchase items to use and sell from vendors.  Some vendors have rebated that are returned quarterly based on our agreement and the amount we purchase.  I see this as a reduction to the cost of a good and would like to put it back to the correct account split up over the months that it is for.  For example, for items I purchased over the 3rd quarter, I will get a check in the 1st quarter of the next year.  I want to break up the amount (let say it is $3000) and credit each month with the correct amount (for example, $1000/month).  This would better show me the true cost of goods that I am buying.  How do I do this if the quarter is closed?  Is there a better way to do this for this type of situation?  I know some people have a rebate account that is considered income, but I don't really see it as income and would like to not use this unless there is another way to use it than just income.

Thanks

Steve