Clearing Payment to a G/L account set up in Reason Code

I am having a problem posting an overpayment/underpayment to a G/L account using a reason code. What I am attempting to do is, using F-28, have the remainder of an underpayment post to a G/L account attached to the reason codes. What I need to have work is that in F-28 I do not want to have to enter any data on what the residual amount is, it should just take whatever is not paid and apply it to the G/L account tied to the reason code. I have set 3 new reason codes in OBBE to test different possibilities with the checkboxes in that transaction, and have attached each of the reason codes to accounts using OBXL. What is happening now is that when I enter a short pay amount for an invoice and I enter a reason code in the RCd field in the residual tab - I hit save and it seems to try to attach the remaining amount to an account that is currently tied to a blank reason code in our setup in OBXL. I would think that since I am placing a code in the RCD field, that it would use the account tied to that. I have also tried to use the reason code field below, but encounter the same results or, with one of my codes that has the "charge off difference via separate account" checkbox checked, it will not allow that code in that field below.

There must be a way to take the shortpay amoun,t and without entering the residual amount anywhere, use the G/L account tied to the reason code to post the remainder to. Can anybody help on what I may need to do to get this to work?

Thanks -

PS. This is not a tolerance issue as I have set the tolerances for both the customer and the user high enough to allow under/overpayments.