In a situation where 2 witnesses would require someone to pay, one witness would require him to swear, but he is not completely denying the testimony of the witness and therefore cannot make the shavua that is incumbent upon him, we apply the concept of מתוך שאינו יכול לישבע משלם. Therefore, when an individual witness testifies that Reuven grabbed something from Shimon, we assume that it belongs to Reuven since he was holding on to it. Had Shimon been willing to swear that he didn't grab it from Reuven, we would believe him against the eid echad. But, since Shimon admits to grabbing it, but is claiming that it rightfully belongs to him, he is unable to make the shavua demanded of him, and therefore must return the item.
Tosafos asks that if Shimon would deny that he grabbed it and make a shavua, he would be believed. Why don't we believe Shimon to say that he grabbed it and swear that it belongs to him, migu that he could say that he never grabbed it? If he would not make a shavua that it belongs to him, it would not be a valid migu since he prefers to use the claim that would obviate him from a shavua. But since he is now making a shavua that it belongs to him, it should be a valid migu (assuming that he can use a migu even if the migu claim would require a shavua d'oraysa)? Tosafos explains that this is exactly the point of argument between rav v'shmuel, against R' Abba. Rav and Shmuel (shavuos 47a) hold that since he has a migu, we don't require him to return the item. But R' Abba holds that even though he has a migu, we require him to either swear to contradict the witness by saying that he didn't grab it, or pay - no other options. Tosafos doesn't clearly speak out the nekuda of machlokes between Rav v'shmuel and R' Abba.
It seems that the point of machlokes is whether an eid echad is mechayev Shimon to support his claim with a shavua, or is mechayev to pay with an option to exempt himself using a shavua. According to Rav and Shmuel an eid echad is mechayev a shavua - meaning, that he is mechayev Shimon to support his claim with a shavua. By Shimon swearing that it is his, and using a migu that he could have sworn that he didn't grab it, he is in essence using a shavua to support his claim. But, R' Abba holds that an eid echad is not mechayev him to just support his claim with a shavua. The fact that Shimon can prove he is correct by swearing it is his in conjunction with a migu, isn't sufficient. The eid echad is mechayev Shimon to pay with the only option out of paying being a shavua to contradict the eid echad. Shimon's only option out of paying is by making a shavua to contradict the testimony of the eid echad by swearing that he didn't grab it. Since Shimon admits to grabbing it and cannot make this claim, we resort to the default that he must pay.