How do you say "if so" in German?
Do you have some time? If so, I would like to speak with you.
Would you say "wenn ja"?
Haben Sie etwas Zeit? Wenn ja, ich möchte mit Ihnen sprechen.
Alternatively you could also say: "Falls ja", "In dem Falle"
As suggested by @Toscho and @Dustin:
And as a more polite Alternative: