Link the Incoming Payment to a Pro-form Invoice

Hi Experts:

We are going to implement SAP BO 2005A SP01 PL29 in a company that needs to link the Incoming Payment to a Proforma Invoice, since in its selling process they receive the money first a gives the customer a receipt or a pro-form invoice and time afterwards they send them the goods with the A/R Invoice including tax (IVA).

How can I manage this?.....In this case, the Reserve Invoice of BO does not apply because it post tax (IVA) account and legally (Here In Colombia) we can´t post taxes for goods that aren´t sell yet.

Thanks very much..

Claudia Patricia Gutiérrez