Mapping the Requirement

[anu_uma]

Hi,
cany any one suggest me to map this requirement????

The client sells goods to his customer
but the payment terms are very different
when he books the sales order , he takes 20% of the amount as advance , when he sends goods he will make an invoice
but this invoice is like this , total amount minus advance taken, minus some percent for retention on condition 1
minus some percent for retention on condition 2 , minus some percent for MRC(After material reaches)
this is the first invoice, then when the condtion of retention 1 is met , then a second invoice has to be raised from system
the same way another 2 invoices are to be raised from system when the retention condtion 2 and MRC reaches.

And at any point of time , the client wants to know , how many retention and MRC invoices are to be generated ....


Pls suggest me how we can do the mapping for this.....?