The Serie A side had been getting closer to extending Smalling’s stay in Rome after an excellent season for the Englishman.
However, United are resisting Roma’s request for a discount and insisting on €25m for 30-year-old Smalling.
The Red Devils would let Smalling rejoin Roma on loan, but only with an obligation to buy him next summer.
Roma’s financial problems mean €25m is too much, despite Smalling’s importance to the team.
As a result, Roma are forced to consider other options and are now focusing on Jan Vertonghen.
Roma would be willing to offer the Belgian defender a two-year deal with the option of a third, worth €3m net per season.
Napoli considered Vertonghen for a while but have made Lille defender Gabriel Magalhaes their priority.
Inter retain an interest but haven’t pushed the button on a deal, fearful he will block Alessandro Bastoni’s rise to the first team.
Taking advantage of Napoli and Inter’s cooling interest, Roma contacted Vertonghen’s agent two days ago to discuss terms.