This is the specifics regarding the debt of Mr. and Mrs. Berl to the Winter Court via Sam Berl’s nixie mother.
They wanted a child. In exchange for that child, or rather, “as soon as the child was no longer theirs” they would forfit the love they had for him.
Sam later (quite by accident) later made an agreement with Puck that he would tell the fae who was looking for him, and in exchange Sam would allow Puck to find out for him which fae had possession of his parents’ debt, with the unfortunate (and unintended) loophole that “should any fae other than a nixie own the debt, I (Puck) will not attempt to take it from them.” Essentially, had Berl been unfortunate, Puck may have been able to obtain the debt for himself.
Berl is somewhat fortunate, though. A green fairy called Abby somehow obtained the debt. Since she is not a nixie, Puck cannot attempt to take the debt from her.