Abaye paskened that Rava Bar Sharshum wasn't able to hold on to the property of orphans and use it as collection, rather he must wait till they grow up and take them to court. The gemara 174a has a machlokes as to why we don't collect from yesomim. Rav Papa holds that the chiyuv to pay a debt is a mitzvah and yesomim aren't obligated in mitzvos. Rav Huna B'rei D'rav Yehoshua holds that we are concerned that the father already payed back the lender, and the yesomim are not aware of that so they can't make a claim to protect their assets. The Rashbam explains our sugya based on the opinion of Rav Papa. That is why they are not obligated to pay until they grow up and become obligated to do mitzvos. Why did Rava Bar Sharshum think that he was entitled to collect even before they become bar mitzvah? Perhaps the Rashbam will hold that since he was already holding on to the property, he has a right to collect from it even though they are not required to do mitzvos.
However, Tosafos explains that Rava Bar Sharshum must have held like Rav Huna Brei d'rav yehoshua that orphans aren't obligated to pay because we are concerned that maybe their father already payed. Rava Bar Sharshum knew about himself that he did not yet collect the debt, therefore he thought he would be allowed to collect from this property. But according to Rav Papa that yesomim aren't obligated to do mitzvos, their is no rationale that Rava Bar Sharshum should be able to collect while they are still ketanim.
Tosafos seems to hold that יתמי לאו בני מיעבד מצוה נינהו, not only exempts the orphans from paying, but creates an issur on the lender and forbids him to collect even from property that is already under his control. Tosafos understands that since they are exempt from doing the mitzvah, it is as if they don't owe the money at all, so the lender cannot collect from them. But, perhaps the rashbam will hold that the concept of יתמי לאו בני מיעבד מצוה נינהו just means that beis din cannot obligate them to pay, since they are not metzuveh to pay. But, since they technically owe the money it would be perfectly fine for Rava Bar Sharshum to collect from property that is already in his possesion since beis din will not have to force the yesomim to do anything.
Furthermore, Tosafos who explains that Rava Bar Sharshum thought he can use the property to collect because he doesn't hold like Rav Papa. Tosafos seems to hold that according to R' Huna Brei drav Yehoshua who argues on Rav Papa, yesomim are בני מיעבד מצוה and therefore obligated to pay their debts. The Rashbam 174a explains this to be their point of argument, that according to R' huna brei d'rav yehoshua they are obligated to do mitzvos. Based on this, the machlokes between R' Papa and R' Huna seems to be whether we view the obligation to pay a debt as a mitzvah which results in an issur, or an issur that is independent of the mitzvah. R' Papa holds that the issur is a result of the obligation to pay, therefore yesomim who aren't obligated to pay, are not in violation of using someone elses property when they don't pay (so beis din doesn't have to separate them from doing an issur). But R' Huna brei d'rav yehoshua holds that aside from the mitzvah which they are exempt from, their is an issur to use the property that they owe to someone else and beis din is obligated to separate them from issur so they cannot allow them to retaing the property, so we force them to pay (if not for the concern that the father had already payed).