Need An answer about an assignment

I have a house on contract to buy I will be doing an assignment most lenders will only allow up to $3,000 to be paid as an assignment or it could kill the deal. Is there somekind of separate contract that can be drawn up with the person I am buying from that can be signed stating that after the closinf they write me a check for my fee. I know most lenders will only a $3,000 assignment fee. I usuall give this to the tilte company so i get me fee after the closing but is there something else i can do with the seller to get like a $5,000 or more fee.