I run a lettings agency and provide different services to my clients. One of the ways we run our company is by advertising these services on our website. At time of advertising we do not own all of these services. When a customer requests the service, we buy the service from the supplier with our own money without charging the customer. At this point, the customer can change their mind and not want the service despite us having already bought it. After we have purchased the service we then charge the customer a higher price. Is this permissible?