How to sell a fractional part of a service?

We arrange tennis competitions. After each month, we got an invoice from the owner of tennis courts. This means we rent the courts. That invoice is based on real usage. The agreed price per 1.5 h per court is 7.50 euros. I entered a service item for that, connected it to the project revenue and expenses accounts and set the purchase price for that supplier. I have no idea what's the standard cost, so I entered 7.50 euros as the standard cost, also. Was it correct?

The problem appears when I want to invoice a participant. For the same service item, I set the same sale price as we don't ask for a profit. If the match was singles, we only sell half of the court rental time. However, after entering 0.5 as the quantity, it will be converted to 1 automatically and the price remains 7.50 instead of 3.75. Some matches were in doubles, so I also need to sale 0.25 of that service item. And any other fractional part of that 1.5 h. How to do that in FrontAccounting?