I too have the same need and willing to pay for development as it will be integral to our upcoming implementation of ERPnext. Typical terms might be 1.5%-15 Days, Net 30; or 2%-10, Net 60; or the lesser common term of 30/60/90 days. These are quite common in USA and a needed accounting function that will automate the payment entry to avoid calculating and entering credits by user ad hoc.