Create a new Scheduled Transaction with the following template:

|Account   | Debit        | Credit |
|----------|--------------|--------|
|Checking  |              | 300    |
|Interest  | interest     |        |
|Liability | 300-interest |        |

When the Scheduled Transaction is entered in the register it comes out as the following:

|Account       | Debit    | Credit   |
|--------------|----------|----------|
|Checking      |          | 300      |
|Interest      | interest |          |
|Liability     | 300      |          |
|Imbalance-USD |          | interest |

Based on help from the mailing lists, it was determined the following works correctly:

|Account   | Debit          | Credit |
|----------|----------------|--------|
|Checking  |                | total  |
|Interest  | interest       |        |
|Liability | total-interest |        |

The articles linked in the FAQ show variables and constants in a multiplication formula. I created a transaction using a similar formula and it worked fine. The only difference I see here is that the variable comes first in the working formula whereas the constant comes first in the example above.