1) The Rashba is similar to this Tosfos. He starts off in the same way, by saying that we could have established the end part of the Mishnah on 5a -- which states that we assume he has not yet paid -- to refer to a scenario where, on the day that he is expected to pay, he said that he has already paid before the time (which we do not believe, because there is a Chazakah that people do not pay back before the time).

2) The Rashba adds a point which is not mentioned in Tosfos -- that this is the same as the Gemara above (5b) which asks on Abaye and Rava that the scenario must be where he says he paid up before the time. Rashba adds that in such a case there is no Migo, as he wrote above (this part of the Rashba is the same as Tosfos).

3) The Rashba concludes that instead of establishing the Mishnah as referring to a case where he claimed he paid before the time (as in the possibilites 1 and 2 mentioned above), the Gemara states a true answer, namely that even if he claims that he paid after the time he is expected to pay, he still is not believed. This is because in the case of the end of the Mishnah, where he leaned a wall against the wall that his neighbor built, he still is not believed to say he has paid, because not everyone knows the Halachah that the Chachamim stated, that if one leans a second wall against the first wall, he must pay for the first wall too. This point of the Rashba is also made by Tosfos.

Kol Tuv,

Dovid Bloom