@Rich Hupper - can you give a more specific location? also, if houses there are selling for 540k, why would you say the ARV is probably 600?
Your name/entity and/or nominee is all you need to write on the buyer line of an offer to purchase to be able to wholesale it.
you can tell him that you find properties for other investors in your network, so while you won't be the one closing on it, one of your partner's will. talking about assignments and fees will generally confuse the seller and potentially cause it to not happen. keep it simple.
That's all you need at this point - you still need to see if he would agree to a number like 400k.
depending where it is, i'd be interested in looking at it Friday with you.
the seller's attorney could hold the EMD once he signs the agreement. also, the EMD can be any amount, so it doesn't need to be huge - if they question it i tell sellers that 5% is the typical amount because in a normal transaction the listing agent puts that amount into escrow so they know they can collect their 5% broker fee. since there's no broker, it just needs to be $1 or more for it to be a valid, enforceable contract.
put a condition in there like subject to buyer's partner's inspection (not a home inspection) or approval
explain that you would like to close in 60 days because you/your buyers get bank financing but will not put a mortgage contingency in there - in other words, you know you'll get financing or wholesale it by that time
if you do get it under agreement, you want to market it immediately.