Employer wants me to sign a new contract during my medical leave

I signed a 10k contract with my company to work for another year and I fell sick and had to take 2 months off.

My employer is asking me to sign for an additional 2 months to compensate for the time I took off and I don’t want to.

I want to complete my contract for the year I signed but my employer says I owe them.

Any advice?

(The contract is to be trained for a new skill and they value the training at 10k)

Thanks