Abraham…spoke to Ephron…saying: "If you would but hear me-I give the price of the field, take it from me, and let me bury my dead there." Abraham, explains Rashi, regretted that he hadn't already paid Ephron. He realized that in the end Ephron would demand payment and so their discussion was futile. Abraham is described as old and advanced in days and the Zohar explains that Abraham made every day count. It is painful for someone like that to watch their time evaporate into pointless discussion and therefore Abraham regretted that he hadn't already paid and moved on.