とのことだ: relaying information received
Distinguish passing on news from another person or organization from direct observation or your own inference.
Present received information as a report
担当者は午後に戻るとのことです。 means that the speaker has been told the person in charge will return in the afternoon. It relays received information rather than describing someone visibly returning right now.
先生は来週まで休むとのことです。 reports that the teacher will be off until next week. The expression can relay information in a relatively formal way, but using とのことだ does not independently guarantee that the reported content is true.
Preserve tense and negation inside the report
戻るとのことです reports that someone will return, while 戻ったとのことです reports that the person has returned. Even with present-form です at the end, the past event expressed by 戻った does not become a present or future event.
担当者は今日は戻らないとのことです。 reports that the person in charge will not return today. If you focus on the reporting frame and omit the negative in 戻らない, you reverse the information being passed on.
Compare reporting そうだ
担当者は午後に戻るそうです。 also reports having heard that the person in charge will return in the afternoon. Reporting そうだ and とのことだ both relay information; とのことだ fits contexts where a received message is being passed along.
戻りそうです is different: it expresses signs or an expectation that someone seems likely to return. Check the preceding forms in 戻るそうです and 戻りそうです to distinguish relayed information from a judgment based on signs.
What information is being relayed in 担当者は今日は戻らないとのことです。?
Show the answer and why
The person in charge will not return today. とのことです marks this as received information being passed on, and the negation in 戻らない must be preserved.