Did you retrieve to the period based on the date of the transaction, or to a specific period. Retrieving based on the date of the transaction is almost always prefered. If this was the option selected, then the transactions will post to the correct period (you can post transactions to different periods in the same g/l batch).

Have you posted the general ledger batch you retrieved from Payables? If not, then inspect the period assigned to the August invoices. If it is your July period, then highlight the August invoices, click the Copy button, Enable the Move check box on the Copy Batch dialog, and move the transactions to a different batch. You can change the fiscal period in this step too.

If you have already posted the batch, see whether the transactions posted to your July or August period. If they posted into August, then all is well. If they posted into July, then the easiest way to correct the error is to Unpost the batch, removing the transactions from the posted file. Fix the batch as described above. Then post the two corrected batches.