Activity <-> Project expense dependency

Dear all,

I've setup a project with several activities. Some activities represent the purchasing procedure of parts needed for the project. The start time is plannified. Now I want to setup a cost estimate for the project. For this I set up project expenses for each part. Since the activity is planned I already have a date until the necessary money for the pruchasing must be available. Is it possible to make dependency between a project expense and a corresponding purchasing activity? This way it would be possible to track the dates when a certain amounf of money must be available.
Or I am completely mistaken and there is a completely different way of handling of planning expenses within the project schedule?

Best regards,

Udo