Cashflow statement ; mapping account

The answer depends on how you are building your Cash Flow.

If using Dynamically computed members, you would simply check the loan account balance and if the sign is as desired then return the amount.  You would do the same in each of the two Cash Flow destination accounts.

If you are using Formula Pass stored calculations in which formulas are attached to each Cash Flow destination accounts, you would need to do something similar as Dynamic calculations, where based on the sign of the source loan account balance, you decide whether to write it to the destination.

If you are using a common Business Rule stored calculations attached to the cube, then you can get the source loan account balance, and based on the signage write the data to the appropriate Cash Flow destination account.

If using stored calculations, there are a few more options that come to mind to accomplish this depending on how you are building the bulk of the cash flow and what sort of things you might be doing to improve performance and/or maintenance.