Maternity rights

I have an employee who wishes to return early from maternity leave.  Initially she had indicated that she was taking at least a year away from work.

The person recruited to cover the maternity leave still has 4 months of their fixed term contract left.

I can't afford to pay for both and I am very happy with the performance of the temp whose performance exceeds that of the employee.

Can anyone advise me on the issues raised by this?