Selling additional leave to top-up furlough amount

Thoughts please! Does anyone know if it’s possible for any employee to sell their additional holidays days (accrued over the statutory minimum of 5.6 weeks per year) and to use this amount to top up their furlough amount? Contractually I’m thinking no but if the employee gives consent is it possible? Heads is frazzled. Could be a good option for them to use up their holiday and to keep a steady/slightly higher monthly income. Thank you! Sarah