Hi,
You declare negative earnings on the employment page SA102. The Self Assessment guidance advises, If you’ve had to pay back money to your employer due to a contractual agreement (often known as claw back), reduce the amount of pay in box 1 by the amount paid back. If the result is a negative amount, enter zero in box 1. Include the negative amount, but as a positive figure, in the ‘Post-cessation trade relief and certain other losses’ box, box 6 in the ‘Other Tax Reliefs’ section on page Ai2 of the Additional information page (SA101). Enter a note to explain why you have done this in box 19 ‘Any other information’ on the SA100. You can see the guidance here:
Employment notes
Thank you.