Whats the criteria for the one related invoice you want to link? most recent? something else? You could create a summary field that chooses the maximum record ID for the child table. This should give you the most recent. However you can add additional criteria like if a field is set to a certain value, then it will be the most recent record that matches that criteria. Then on the summary field, choose to link to the related record. Place that summary field somewhere on your form on the parent table. Make sense?
