Hide a value in a cell unless another cell has a given value

I'm trying to create a formula to generate a list of values in a dropdown.

1. Each row contains a task that has a status, and a priority.
2. The priority is set from a dropdown containing a list of numbers from 1-50.
3. No two tasks can share the same priority so the dropdown must exclude any already in use.
4. When a task has a status="done", its priority is no longer considered in use and should once more be available in the dropdowns.

For example, in the table below,

• the dropdowns are in column A
• The priority 15 in A4 should not be excluded from dropdowns because C4="Done"
• Therefore, the dropdown in A6, for example, could include 15 but would not include any of {24, 32, 49}

I'm able to eliminate any assigned priority from the dropdowns using the following formula, but I can't get to the next step, restoring used values to the dropdown if they are assigned to a task with status="done":

=ARRAYFORMULA(   IF(     COUNTIF(       H2:H1000,A2:A1000)=0,       H2:H1000,))

How would I reinstate a value that was used, such as in A4 of my example, if the status of that row is set to "Done"?