Need help for the formula

Hi,

I am having 2 sets of data for sales and stock. Between Sales and stock common field is Style. I need to find the SKU for the sales. Style id has more than 1 rows in both data. If we have mention the 1 sku in sales so for the next line of same style id i required the 2nd sku no. I am attaching the sheet where sales data sku column is blank. Sku no should not be duplicated in sales.

Thank you