Paypal donation tracking system

I am looking for a donation tracking system for my free fonts site. Basically I want users to be able to hit a "Donate" button, and make a donation (simple enough using PayPal's own system I know) BUT what I then want is, based on a date period and amount, to be able to display, once the donation is made, how much is left to find for the remainder of the period. So say in January $200 is needed, a user donates $20 and the screen refreshs to show that "$180 is still needed to cover January's costs". I have a similar system running on a phpBB forum but am unsure if this is a mod specific to phpBB or if it can be modified further to run on a non-phpBB site. Bidders simply copying and pasting replies will not be considered. Ask any questions necessary via the PMB system please.