Got my first short sale under contract! Need clarification on assigning it

I'm a realtor and beginner investor and understand much of the process of short sales, but not the intricacies. So I got my first short sale under contract with a seller and we're assembling the short sale package for the lender. I have a fully assignable contract and have a question or two as to exactly how to proceed. Once I find a buyer and execute the assignment form do I collect my assignment fee at that point or do I get paid at closing through escrow? If the new buyer is financing the deal, I've heard that their lender may not like the whole assignment thing and ask you to restructure the whole deal. If it's a cash buyer I don't see as many potential problems. Any other advice would be greatly appreciated regarding assignments or assembling short sale packages. Thanks so much!