The רבנן said he who comes to collect, etc. - אמור רבנן הבא ליפרע כולי
רב"ש maintains that it is on account of the חשש צררי that one may not collect from יתומים קטנים even with a שבועה. Therefore since רב"ש knew there was no צררי, he (thought that he) was able to collect the debt from the estate of the יתומים קטנים and would be able to circumvent the שבועה through the מיגו of לקוחה היא בידי.
This is how we read the text; eventually he admitted to him - הכי גרסינן לסוף אודי ליה
[Either קריביה admitted and granted the field to ראב"א or] ראב"א brought witnesses that he was a קרוב and therefore since the field legally belonged to him, קריביה had to pay for the פירות despite his מיגו (as ר"ח maintains). It is not comparable to the ס"ת case where the סופר forfeited his wage.