It is no problem to make bonus payments. These are additional payments you can make beyond the normal payment for a participant's submission. You can use these to allow for variable rewards, prizes, or to make partial payments to participant who have not completed your study but you still want to reward.
Simply go to "View Submissions" - > "Bonus Payments" from your studies dashboard. You can then submit a csv with the details of the participant id (which you can collect during your study) and the bonus amount.
Please note that a processing charge is applied on top of bonus payments.
If you are using the new site https://beta.prolific.ac/ you can make bonus payments by selecting this option from the "More" dropdown while Viewing submisisons for your study.