Write-offs - R11.5.10

I am new to the receivable application and I have a request below;

Our current small balance write off in Oracle is $19.99. We need to increase this from $19.99 to $50.00.   Please make sure we are not adjusting off small dollar invoices with the original bill less than $50.00. We only need to adjust off items with a $50.00 balance or smaller to invoices which have a variance of $50 or less between the original balance and the open balance.

I am not sure where to go to determine how to change the write-off from 19.99 to 50 (only the difference between original balance and open balance).  Can someone provide documentation other than the standard Oracle Receivables user guide?  Or provide a path in the right direction.