Change registration fee after registration

Hi Tuan,

I need to update the registration fee of one registration. Today he registered with paying 890€.
Now, I need to give him a discount of 100%. So he has to pay 0€.
I am doing the following steps:

1. In field Discount Amount I put in 890€
2. I click on "Re-calculate Registration Fee"
3. Save
(see first screenshot)

After saving, nothing changed
4. Discount Amount is empty
5. Gross Amount is showing 890€ (as bevore)
(see second screenshot)

What is wrong?

Thanks and regards,
Thomas