The essence of the issue the Talmudical Rabbis had with Rabbi Tarfon redeeming and subsequently returning the first born redemption money hinges on the Rabbis’ lawful opinions of such a practice.
In their first statement they are demonstrating their legal concurrence with his practice.
Rabbi Eliayhu Russof in Yad Eliyahu (note 2) discusses the ramifications of the Rabbis’ concurrence. For what is the question, really, that the Rabbis discussed whether he fulfilled this or not. Because had he not returned the redemption money afterwards he would have certainly fulfilled this properly.
At first glance one could think that if he didn’t have in mind to return the money, and then later returned it after the fact, this would have been an appropriate way to return the money. As apposed to having a pre-thought to return it, which would not be a plausible strategy.
However, this too is inconceivable. For after all, once he has accepted the money it becomes his, and he may do with it as he pleases. But since Rabbi Tarfon is cited as ‘this is how he used to deal etc.’, there is an implication that he certainly had in mind to give it back and thus the father’s ‘giving’ of the money may have never been valid at all.
The answer then must be proved from a similar anecdote in Baba Kama 50a, which discuses Neḥunya’s scheming in giving free wells to the public.
ת"ר חפר ופתח ומסר לרבים פטור חפר ופתח ולא מסר לרבים חייב וכן מנהגו של נחוניא חופר בורות שיחין ומערות שהיה חופר ופותח ומוסר לרבים וכששמעו חכמים בדבר אמרו קיים זה הלכה זו הלכה זו ותו לא אלא אימא אף הלכה זו:
The Sages taught: If someone dug or opened a well and transferred it to the public for their use, he is exempt from damage caused by the well. If he dug or opened a well and did not transfer it to the public, he is liable. And this was the practice of Neḥunya the digger of pits, ditches, and caves, who would dig, open, and transfer them to the public so that there would be wells of water for public use. When the Sages heard about the matter, they said: This individual has fulfilled this halakha. The Gemara asks: Only this halakha and no more? Isn’t it well known that Neḥunya was a great man? Rather, say: He fulfilled even this halakha and did not forget to transfer the wells to the public.
It is obvious from Neḥunya’s conduct that had in mind to take care of the pits if the public would not accept responsibility over them. And he actually accepted upon himself an intrinsic responsibility to cover them up and the likes, should the public reject such responsibilities.
Such is the nature of the case regarding Rabbi Tarfon’s practices of fulfillment of the law. He had in mind certain conditions that he is accepting the money such that it is qualifies for the father’s obligation. However, his stipulation extends that if he wants to give the father a gift later on, he may do so out of the goodness of his heart and with no interposition to the father’s obligation.
It thus comes out that Rabbi Tarfon and Neḥunya followed a pious mindset, and did not want to benefit from others similar to the practice mentioned in Yoma 39a.
מכאן ואילך נשתלחה מאירה בעומר ובשתי הלחם ובלחם הפנים וכל כהן מגיעו כפול הצנועין מושכין את ידיהן והגרגרנין נוטלין ואוכלין
From then onward, a curse was sent upon the omer, and to the two loaves, and to the shewbread, that there were not sufficient quantities to give each priest a full measure. Therefore, each priest received just an amount the size of a bean; the discreet, pious ones would withdraw their hands, a bean-bulk being less that the quantity needed to properly fulfill the mitzva, and only the voracious ones would take and eat it.
Finally, the wording used ‘that he has nonetheless fulfilled the law’, is used to tell you that this teaching was brought to learn that both Rabbi Tarfon and Neḥunya fulfilled the essence of the law and that we may apply their practice in practice.
The rest is just Talmudic idiom; that should be obvious.