At any point you can sell the lease, at a price that is agreed with the Landlord. You will have to pay your landlord a commission on the sale of 20%. Any new buyer will be responsible for the costs involved in the sale.
Important
Your lease is a legal contract and is governed by Spanish law. If you have a disagreement with your landlord, either party can take legal action in the local court.
You have rights as a tenant and your landlord has rights to protect his property.