Set the Accounts Receivable Options to use the Invoice date as the Line Item post date:

1. Select Tools, Options from the menu bar in Accounts Receivable.
2. Select the Records tab.
3. Highlight Invoice Line Item on the left-hand list.
4. In the Default Line Item Date field on the right, select to use the Invoice date.
5. In the Default Line Item Post Date field on the right, select to use the Line Item date.
6. Click OK.

OR 

1. Open the Invoice, go to the Line Items tab
2. Open the line item with the incorrect Post date, change the post date
3. Save and Close