Following the massive cyberattack on UnitedHealth Group's Change Healthcare unit last year, the company launched a temporary funding assistance program to help medical practices with their short-term cash flow needs, offering no-interest loans with no added fees. A little over a year later, UnitedHealth is aggressively going after borrowers, demanding they "immediately repay" their outstanding balances, according to documents viewed by CNBC and providers who received funding. Some groups have been asked to repay hundreds of thousands of dollars in a matter of days. Optum, UnitedHealth's financial, pharmacy and care services arm, is telling borrowers that it reserves the right to "begin offsetting claims payable" to the practices, meaning the company will withhold separate funds until it recoups the loan.