Synonym, but this is about context
Not to condescend, but we should know out the door that this is a very good, beginner level question. When we don't find understanding, we tend to look for meaning in a single word, even if that word won't satisfy our curiosity. In hermeneutics—Biblical or for other literature—looking intensively toward one word indicates that we might benefit by looking at the larger picture, and also at the original language.
Since this is partly a translation question, it was appropriate for the OP to look at the original Greek. Let's take a deeper look...
Rev 5:3-5 (Nestle-Aland 26th, emphasis added)
3καὶ οὐδεὶς ἐδύνατο ἐν τῷ οὐρανῷ οὐδὲ ἐπὶ τῆς γῆς οὐδὲ ὑποκάτω τῆς γῆς ἀνοῖξαι τὸ βιβλίον οὔτε βλέπειν αὐτό. 4καὶ ἔκλαιον πολὺ ὅτι οὐδεὶς ἄξιος εὑρέθη ἀνοῖξαι τὸ βιβλίον οὔτε βλέπειν αὐτό. 5καὶ εἷς ἐκ τῶν πρεσβυτέρων λέγει μοι, Μὴ κλαῖε: ἰδοὺ ἐνίκησεν ὁ λέων ὁ ἐκ τῆς φυλῆς Ἰούδα, ἡ ῥίζα Δαυίδ, ἀνοῖξαι τὸ βιβλίον καὶ τὰς ἑπτὰ σφραγῖδας αὐτοῦ.
The word οὐδεὶς (oudeis) means "no one" in the sense of "no person", v3. And, εἷς (heis) from v5 means "one" in the sense of "one [person]", refering to "one" of the elders who addressed John. This is very similar to English use of "one" in place of "someone".
Translating the word as "man" would not change the meaning whatsoever. I defer to BDAG and Kittel for studies on that matter, but this is not the main question.
The main question was whether it is a contradiction to say Jesus opens it, thus Jesus can't be a "man" since no "man" can open it.
Is this a contradiction?
No. This is mainly because the purpose of this passage is not to develop an ontology of Jesus's character. It's a narration of events.
A larger question of contradiction would not be about using "man" vs "one" to mean "person", but the fact that vv3-4 records that there wasn't anyone worthy, but v5 reports that there is someone who is worthy. So, is there someone worthy or not? First there wasn't, then there was. That's not a contradiction; that's a sequence.
First, no one was found worthy, then in that narrative context, the Lamb enters the scene, and he is actually worthy.
Consider the author's style
This is also consistent with John's narrative style, which makes his Gospel different from the Synoptics. Almost everything that happens in the Gospel of John merely sets the stage for Jesus to start talking and acting. Once Jesus shows up in any scene, all other characters seem to fade into the background and may not be referred to again. This is similar.
No one in heaven, earth, or below could open the seals. Then, enter Jesus. Translating "no one" as "no person" or "no man" would not alter that meaning whatsoever.
Excellent question! Many are curious about such things.