Amrabat spent the 2023-24 campaign on loan with Erik ten Hag's side, making 30 appearances in all competitions, including 21 outings in the Premier League.
Under the terms of the agreement that took Amrabat to Old Trafford last year, the Red Devils were able to sign the Morocco international on a permanent basis for €20m (£16.9m) during the current market.
There has been a host of speculation surrounding the midfielder's future in recent weeks, but according to Fabrizio Romano, a decision has now been made by the 20-time English champions.
The transfer expert claims that Man United have told Fiorentina that they will not be activating the buy option.
Man United 'will not pay €20m (£16.9m) for Amrabat this summer'
However, the Red Devils "remain interested in Amrabat" and are keen to discuss a separate deal, with Ten Hag believed to be keen to keep the Moroccan.
A number of other clubs are believed to be keeping a close eye on the situation, with the 27-year-old set to leave Fiorentina this summer, as he only has one year left on his contract.
Amrabat had difficult spells during the 2023-24 campaign, but he impressed in the latter stages of last season and was chosen ahead of Casemiro for the FA Cup final against Manchester City.
The midfielder featured in each of Man United's last six Premier League matches of the season, meanwhile, starting the last three against Arsenal, Newcastle United and Brighton & Hove Albion.
The Red Devils are still looking to bring in another midfielder, with Paris Saint-Germain's Manuel Ugarte their leading target, while Real Sociedad's Martin Zubimendi is also thought to be an option.
What has Amrabat said about his future?
"Do I already know what I'm going to do next year? No. We are going to talk. Staying is certainly an option," the midfielder told Ziggo Sport after the FA Cup final.
"Look, I think it has been a very difficult season. For Manchester United it needs to be much better, and also for me personally."
Man United have already allowed Donny van de Beek to leave this summer, while Christian Eriksen is widely expected to move on in search of regular action.
Scott McTominay and Casemiro are also being linked with exits, but Ten Hag has said that he wants to keep the former at Old Trafford amid the speculation surrounding his future.