While getting rid of all university staff and rehiring them on different contacts is an option, the university said it was a last resort.
The plans are one option in a consultation to address a huge black hole in the company's budget caused by the possibility of reduced student numbers in September.
Unite said that the university bosses were looking for staff to go down to four days a week, with no pay rises, no incremental pay rises and no promotions, and the union would "campaign strongly" to protect jobs.
A spokesperson for the University of Sheffield said the impact of the Covid-19 pandemic meant it did need to make some cost savings in the next financial year.
"We have done everything we can to identify savings in areas of non-staff expenditure first, including stripping back our planned capital projects, and have offered a range of voluntary measures for staff, including a voluntary severance programme and the option to reduce hours or purchase more annual leave.
“As a next step we have begun a 90-day consultation on some potential temporary changes to some of our terms and conditions of employment. This consultation does not mean that the University will definitely need to make these changes and we will do all we can to avoid compulsory action, but it is important that we begin honest discussions with the trade unions who represent our staff in case we need to do so in the future.
"We recognise that this is an unsettling and challenging time for everyone and deeply value the continued efforts of colleagues across the University in preparing to welcome our students in the next academic year.”
Unite regional officer Harriet Eisner said: “We are appalled at the callous attitude that the University of Sheffield has displayed to its dedicated workforce. It is a wealthy institution with a lousy business model.
“The announcement is predicated on a shortfall in students this coming academic year – which necessitates this call for a £100m in savings. The university’s management are panicking that their top fee paying Chinese students will defer, but we have not seen any evidence of this.
“It is clear from its proposal – to reduce pay and terms and conditions for its employees – that it has no strategy. Instead of working with unions to create a more sustainable strategy for the future, the university intends to strip back the terms and conditions of its skilled workforce.
“The threat of dismissal is no way to enter into consultations with union representatives about their members’ basic terms and conditions.
“The University of Sheffield has a hopeless business model which relies on attracting top fee-paying international students to prop up an institution which is a major employer in the city.
“The leadership of the university need to think very carefully about why it wants its staff to pay the price for the top management’s failure.
“Unite will be challenging any cuts to terms and conditions. We will work strenuously to protect the jobs of our members and wish to engage constructively with the university to chart a positive way forward.”