This is not currently possible in OJS. Only the non-manual payment methods may be used to purchase individual articles.
We have, however, flagged it as a potential enhancement for a future release. You can find the bug report here
. Please feel free to cc yourself and/or add any additional details to the description.
If you have direct access to your OJS database, you could manually add the necessary records. In effect, you would need to add a new row to the completed_payments table for each user/article that you wanted to grant access to. Please see classes/payment/ojs/OJSCompletedPaymentDAO.inc.php for more details, in particular the insertCompletedPayment and hasPaidPerViewArticle methods.