Currently a payment can be deleted until it has been sync'd to your accounting system. If it has been sync'd, the payment must be 'refunded' so that the connection to accounting can make the correcting entries in accounting.
If you want to adjust the payment amount, if the payment has not been sync'd to accounting, you can delete the payment and add a new one. If it has been sync'd to accounting, then you must refund part or all of the payment to make the adjustment.