For instance that he made it into an אפותיקי, etc. - כגון דשוייא אפותיקי
Initially תוספות assumed that (even) in a case where the מלוה is owed כשיעור ארעא ושבחא and it is an אפותיקי the מלוה must pay the יציאה (as a יורד לתוך שדה, etc.), and proves it from יתומים. However תוספות reconsiders and differentiates between יתומים who have no recourse (and therefore the מלוה compensates them for the יציאה) and a לוקח who has recourse (so the מלוה may not be required to compensate him). [Selling an אפותיקי field is not the same as selling a stolen field.]
He has money; but no improvements - מעות יש לו שבח אין לו
In a case of הכיר בה שאינו שלו there is a difference whether there was אחריות [in which case the buyer is compensated for the שבח], or there was no אחריות [in which case the buyer is not compensated for the שבח (and not even the קרן according to שמואל)].
And he gave it to him for a present - ונתן לו לשם מתנה
The concept that if we know that a sale is invalid therefore we assume that the ‘purchaser’ intends to grant it as a gift to the ‘seller’, is valid only if it is very obvious that there can be no sale (as in the case of הכיר בה); however if there is a possibility for the buyer to [mistakenly] assume that there can be a sale (as in the case of מוכר בשנת היובל), then this assumption does not apply and the monies are returned to the buyer.
דמית לוקח means, according to רש"י, that he died after the גזלן purchased it, and according to תוספות that he died beforehand. If he died afterwards, according to תוספות (everyone agrees that), it would remain by the heirs.