Credit system to use with a schedule

Hi there

We are building a site which sells coaching sessions.

We would like to set up a credit system, which the customer can then use their credits to buy slots on our training calender. Is there anything out there straight out of the box that does this? I understand I could use a credit system and link it to a shop with the lessons as products, but it seems a little archaeic and I would like something more appropriate.

Thanks in advance.