Dynamics 365 - IT HelpDesk and Invoice automation

We need a way to manage Dynamics 365 cases and it's billable time in when they are solved. In one hand we've got clients that contract a "bag of hours" (I don't know how to translate it to English, but I think they are known as "rights" in Dynamics), so the resolution time of the cases should be discounted from the hours they have contracted and when these hours are consumed, the system must generate an invoice for the client to contract another ammount of hours. In the other hand we've got clients who pay monthly for the ammount of hours that we work for them. So we need to automatically generate an invoice monthly from the total total time of the cases asociated with each of those clients. Our billing and invoice system is [login to view URL] and we need to connect both systems via API so the invoices generated in dynamics are also registered in holded.com. This is what we are need for now, bur we are planning to make further changes depending on the needs of our workflow. Sorry for my bad english. If there is any doubt, feel free to ask or whaterver you need. Thanks