Deleting fields used by roll-up summaries
Key Takeaway
You can't delete a field that a roll-up summary definition references, such as the field being aggregated.
A roll-up summary field sits on the master side of a master-detail relationship. It shows a value on the master record, such as a count or total, calculated from the related detail records. Because a roll-up summary field depends on fields on the detail object, it limits which fields you can delete. You need to know what blocks a deletion and what happens to a field after you delete it.
What a roll-up summary field calculates
A roll-up summary field can COUNT the detail records. It can also find the SUM, MIN, or MAX of a field on those records.
For example, a Total Invoice Amount field on Account can sum the amounts of related invoice records.