All FI postings from SD will happen based on Account Key you assign to your condition type in pricing procedure which will then be linked to G/L Account assigned in VKOA. So check your pricing procedure for return sales what Account Key is maintained. Next go to VKOA and check for what combination, G/L Account is assigned. If you want to create a new combination for returns, you can do so via SD => Basic Functions => Account Assignment Costing => Revenue Account Determination => Define Access Sequences and Account Determination types
G. Lakshmipathi