See, here mainly you are using two accounts for debit
1. GR/IR clearing account
2. Stock account (G/L which you've used in AAC tab in PO)
In your case, you've done GR for amount 503127.09. So the GR/IR account credited as 503127.09.
Then you have done IR and vendor credited and GR/IR account has been debited.
So now your GR/IR account is nil. Now you are doing again IR for some amount.
As because you haven't posted GR yet (For the next IR), So system is taking the amount from Stock G/L account instead of GR/IR clearing account.
And how system is taking the account?
It is a internal process as like as when system found a price difference system takes the amount from PRD transaction key - G/L account by using value string.
