>CboClient_ID : John Daves -----> all the payments of John Daves(in Grid´s control)
>cboClient_ID : Lucas Arnold -----> all the payments of Lucas Arnold(in Grid´s control)
>
>next client(>) = next payment(s) of this client
>prior client(<) = all the payments of this client
>last client(>|) = all " " " " "
>first client(|<) = all " " " " "
>
>In other words when user navigates the table I want that all relational payments
>appears with your own client.

This should be working. How are you defining the relations between the tables? Are you only using the grid for the relation or do you have relations also set in the dataenvironment? I have had problems when I have set the relation between tables with both (maybe this shouldn't be a suprise). If you have done this remove one of relations in one of them.