"Sir, the ***** owes me money and this is the only way I know for sure I'll get it or something close to it in trade. I know this may sound terrible but trust me when I tell you the last 6 girls who owed me money I let slide and now I'm out the moola. So what do ya say? Can I have her hand in marriage?"

I'd take above template and work from there.

In all seriousness though I'd just state you intentions and be honest with him, it is up to her but asking the family shows respect. I had to ask my wife's parents as well. No biggie just honesty and straightforwardness.