I got trouble in booking payment on partially credited invoice.

In "Purchases/Payments to Suppliers" I select a supplier that returns an empty list of dued invoices.
In "Purchases/Allocate Supplier Payments or Credit Notes" with "Show Settled Items" checked : returns a negative "left to allocate" amount.
I.e.:tax (vat) inclusive Invoice : 1.000 / Credit : 200 : should be 800 => is (-800)

In table:supp_trans / field:alloc I see 1.000 instead of 200 !
same as in table:supp_allocations / field:amt 1.000 instead of 200.
I tried this correctioin in db and everything looks good.

I think, I'm right but... Am I really ?
How to solve this ?

Thanks in advance.