How do you correct inventory that was entered on the inventory sku instead of creating a bill without causing duplicates?

Under inventory, my husband created SKU for the parts that he is selling with the repairs. Up until recently as we were buying new parts, we were not creating a bill and adding the individual items.  He was going into the inventory screen under products and services, pulling up that item (SKU) and adding the quantity he purchased. We were then creating an expense for that vendor with the total dollar amount we paid and listing it as parts. we have since realized our error and would like to correct what we entered wrong without causing the inventory counts to be way off. Please advise how I can correct these. Thank you for your response.