Need Help to Manually override a field value that is set by a Business Process

Hi,

I have 2 fields in Dynamics (Sales Stage and Probability). Sales Stage is the Driver for the probability field. Using a Business Process, I have set up the following:

Sales Stage         Probability

Prospecting         25%

Needs Analysis    30%

Closed Won         100%                

For each Sales stage that is selected, the business process runs which then feeds the live probability %. 

The issue I am facing is sometimes we want to manually change the probability. So for instance, an opportunity may be in prospecting Stage however instead of 25% we want to manually enter 40%. When I do so, it just saved back to the default of 25%.

Is there a way of doing this?

I am using Business Process because I noticed when I use a workflow, it only updates the probability field once the form has been saved and refreshed. 

Thanks in Advance