If the business model does not allow for a full sell of the software (as suggested by someone, but it implies higher up-front costs for the buyer company) and doesn't fall in the Open Source "support" model, one can sell it on a time based license, making it clear in the contract and including also clear schedules for:

• expected renewal of the license;
• end of service if a new licence is not agreed in time (which should be a fair amount of time after the expiration of the first bullet).

In this case, I'd encapsulate some central functionality inside a XS module, together with the limiting code.

The OP should also consider how long all this should go on before they acquire the right to use the software without limitations. There are softwares (like a popular GSM planning tool I once used) that are basically "rented" for limited times, but this timed scheme could just be used to divide the price that makes *the OP* happy in chunks that make *the customer* happy.