i had create a collection journal linked to the same cash journal cash account.

i had create a pay later  journal linked to account receivable account.

i had create a service product called collection with Zero price.

when collection of 100$ only fix the customer and new  invoice with QTY=1 of collection product and

set collection journal to 100$

set paylater journal to -100$

total is Zero but it fix customer balance cash account 

also report in POS the 3 activity (cash sales- pay later sales - collection )

:)