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The link is generated by PayPal and it is not possible to get this link, save it and go back to complete the payment in another time since this is temporary available and may be that when you try to load the payment link it is not available anymore.
Once you have completed the payment manually your submitter will need to submit the form again or pay you directly, here is a link with more info about incomplete payments on PayPal: Paypal-Incomplete-Payments-FAQ
I would suggest you to enable IPN on your PayPal account as well, here is a guide about that: -How-to-Enable-IPN-for-Paypal