A legal wrangle over the interest rate that applies to loans provided by Columbus Capital (and before that ANZ) to a mortgage manager may turn, in part, on whether Pioneer Mortgage Services acquiesced to unilateral changes in terms dictated by the bank several years ago.Earlier this year, Pioneer Mortgage Services sued ANZ and Columbus Capital over fees Pioneer claims it is owed by the bank and Columbus. Pioneer, a Gold Coast-based firm, contends that Columbus is demanding additional payments of around A$25,000 a month in the form of an increased "delivery rate".Pioneer is seeking a declaration that the correct delivery rate is the bank bill swap rate plus seven basis points and 75 basis points on all loans "introduced and managed by the plaintiff that are [being] funded by ANZ."Columbus, which bought the Origin mortgage funding business from ANZ in September 2012, lifted the delivery rate to a margin of 160 bps over the bank bill rate from December.Legal action by other aggrieved mortgage managers subject to the same changes is rumoured to be in the works.ANZ, in a response filed with the Supreme Court of New South Wales, asserts that Pioneer "by its failure to inform [the bank] of its allegations… represented that it did not dispute [ANZ's] right to vary the delivery rate from time to time."ANZ says it notified Pioneer in 2005 of its new pricing model.The bank says that it and Pioneer both relied in their normal business dealings on the "assumption" that Pioneer did not dispute the bank's right to vary the rate.In essence, ANZ is claiming that it told Pioneer Mortgages that Origin had the right to review the methodology used to create the rate and that Pioneer didn't complain last year when the rate changed, so Pioneer must have agreed with it.