How to deduct a tax rebate from an roi calculation?

[Nea Zăpadă]

Hi,

I'm trying to figure out how to make the roi calculation discount a tax rebate. Say I make a $1000 investment into some tax advantaged investment vehicle. Then my tax authority returns 20% of that, so I get $200. I want the Cashflow to show as $800, because that's the amount that I'm actually risking on that investment. I've taken out the $200 and used it for something else. The value of my investment should still be $1000.

I've tried a few things, but here's an example:

2023/01/01 invest
    Assets:Cash
    Assets:Pebbles                          USD 1000

2023/05/01 tax rebate
    Assets:Cash                              USD 200
    Income:Tax Rebate

2023/07/01 investment income
    Assets:Pebbles                           USD 100

    Income:Pebbles

$ hledger -f example.journal roi --inv Assets:Pebbles --pnl Income:Pebbles
+---++------------+------------++---------------+----------+-------------+---------++--------+--------+
|   ||      Begin |        End || Value (begin) | Cashflow | Value (end) |     PnL ||    IRR |    TWR |
+===++============+============++===============+==========+=============+=========++========+========+
| 1 || 2023-01-01 | 2023-07-01 ||             0 | USD 1000 |    USD 1100 | USD 100 || 21.06% | 21.06% |
+---++------------+------------++---------------+----------+-------------+---------++--------+--------+

In the report above, I'd like Cashflow to be USD 800 and Value (end) to be USD 1100.

What can I change either in the way I'm recording the transactions or the report command in order to get those values?

Thanks!