Hi Oliver,
Create a new condition type as "interest on adjusted rent". Use the formula as % Share in condition".
After you have completed the adjustment of the condition increased you have to add a new statistical condition in the same period with the amount that is increased (As I suppose the interest would be calculated only on the increased amount).
In the calculation tab of the condition "Interest on adjust rent" created in the previous step select the statistical condition and add the % - Share with the interest percentage.
Now you can post it as one time rent or actual rent as per your requirement.
Best Regards,
Hardik Sharma