Account determination for condition based down payments

Hi experts,

please help me with down payment procedure, which I try to set according this link http://help.sap.com/erp2005_ehp_06/helpdata/en/46/27dcd449fc14dce10000000a155369/frameset.htm

So we make SO with AZWA condtion

enter down payments in F-29 with reference to SO number, Item number and schedule line

Then Vl01n, GI

When I create billing doc. using VF01 I expect to see down-payments received for SO in AZWB condition, but AZWB is empty.

When I fill AZWB value manually it is not posted in accounting.

So, my questions are:

1) Any ideas why AZWB is empty? I use formula 49 for calculation. Can this error be caused by some wrong settings for item category of billing doc?

2) What settings should I make, for create postings in accounting, that described on help.sap.com? Should I assign special accounting key in accounting determination for AZWB condition?

Would be very grateful for answers,

BR, Ivan