Invoicing by Quote/Estimate not time spent

Hi

We invoice in 15 minute increments. An employee spent 0.15 hrs on a task; however I want the invoice to read 0.25 hrs and charge (as quoted). But the employee time to remain 0.15. Can't seem to figure out how to make this work.

Similarly i would have the opposite problem if they spent 1.00 hrs on a task and the quote was for 0.5 hrs, the invoice would need to read 0.5 hrs with the employees time remaining as 1.00.

Please point me in the right direction. Clearly I can't override the invoice... or can I?

Thanks
Kristen