Adjusting Item Receipt Values After Period Close

Here's my scenario: Items were received at $10/unit. The period closes, and then we get the vendor bill and the vendor is charging us $15/unit. The need is to not just post a variance, but to actually adjust the value of the existing inventory. Also, in the case where there are multiple transactions going into our value/average cost calculation, we don't want to go through and recalculate the entire value of the inventory to do an inventory worksheet

My initial though was to do an inventory adjustment, with 1 negative line for the IR qty and value, and 1 positive line at the corrected value. Basically just correcting the one item receipt. The problem here is, apparently you can't change the unit value on the negative line (which makes sense).