How can user overwrite derived zero with a hard zero on a web form?

Dear experts,

Seemingly easy question, but this gives hard times to our users:

There is a Balance type account for the headcount of the entity. Many entities in our group have zero employees and we requre them to confirm it with a hard zero on this account (there is a validation based on Hs.GetCellRealData). However when users open the web form in a new period they see a grey zero derived from the prior period and if they try to overwite it with hard zero they cannot submit it (because for HFM there is no change in value so no need to submit). As a workaround users need to first enter and submit