Honestly, it already sounds like this guy is going to be a MAJOR pain to deal with. I'd avoid the situation, but, that's just me.
IMHO, it is quite rude for someone to ask for money up-front. I mean, unless there is a TON of history involved in this site, you have absolutely no idea if you'll pull ANY keepers from the ground. So, paying him up-front would leave you in the hole. A 50-50 split is a MUCH better arrangement. However, it sounds like this guy is trying to strong arm you.
Unless it is a very small amount, say fifty bucks or something, I would avoid this guy like the plague. Oh yeah, who's to say if you DO pull up something nice (and valuable), that this person won't look to start changing the plans.
If you do indeed go ahead with this one, I'd most certainly have everything in writing.
Most people will give you the go-ahead to hunt their property with no strings. Out of the kindness of their own heart. Sometimes, they will ask for a split of the finds, or, to return any valuables or keepsakes belonging to the family if you found them. However, this guy sounds like a crook.
Just my two cents
