Thats a nice alternative, the only problem is that its hard to be precise.. :/
If you're using MGE, you can bind keys to several Haggle macro functions and precisely adjust amounts in increments of +/- 1, 100, and 10000. Each adjustment requires a separate key bind. Additionally, recent beta revisions of MGE v3.8.2 add functions for Haggle +/- 100 and 1000.