We are a charity- Is it possible to use a simplified sales process?

Hello,

Currently we are using a simplified version of the sales pipeline process.  For example our revenue comes from donors instead of customers.  Fundraisers (Sales Reps) create opportunities on prospective donors and close the opportunity by changing the status of the opportunity to 100%. 

The problem we are having is that when the donor submits less than the original projection, the fundraiser goes back to the original opportunity and adjusts the projected amount to reflect the amount that was actually donated.  The difficulty is that this doesn't render an accurate reflection of what the fundraiser projected and what the donor ended up giving.  There is then a major discrepency between what the fundraiser projects and what the actual forecast is.