The participant decided to leave your study early, or withdraw their submission after completing your study. They have therefore returned their submission, so that their Prolific Score is not penalised with a rejection.
No further action is required, as these submissions are automatically excluded from your total number of requested submissions. You will not be charged for this submission unless you choose to manually approve it (by selecting the green tick).
Researchers cannot return submissions. If you want to offer partial payment to participants you should request that they return their own submission, by sending them a message. They can do this from their Submissions page, by selecting the red circular arrow to 'Return + cancel reward'.