Database Design: Normalization and NULL values
If ResponseToTransactionId is always NULL for items marked 'REQUEST', and contains the 'REQUEST' items ID where the status is 'ALLOW' then It means that from an application point of view you should always be able to locate which records are linked by this field. 

And if NULL then you know there is no link.

It is a bit unusual to do it this way, but it should be fine