M. Hewings in his book Advanced Grammar In Use explains that the modal verb will in the original statement, suggestion, etc. can be either changed into would or unchanged i.e. still will when it's used for existing or future situations and past tense verb in reporting clause.
In the exercise, however, the usage of the modal verb will seems to be changed into would. The sentence is shown below
Underline the more approproate verb. If both are possible, underline them both
- Inge promised that she will / would be at home by nine, so I phoned her shortly after that.
The answer key says the only possibility is would, that means using will won't appropriate, but doesn't it contradict with the explanation above that I can use either will or would? Or is it possibly that the situation in the sentence is neither existing nor future?